DON'T use an impact to remove or replace the driving pinion nut. (The Ford Shop Manual also warns against this). The rapid, cyclic impacts will vibrate the roller bearings on the drive pinion. This can cause brenelling (small pits) of the surfaces of the roller bearings and races. Once this occurs, the bearings and races will begin to degrade and deteriorate.

Here's how I have removed/reinstalled the yoke to replace a driving pinion seal. Take a paint pen and make a line across one half of the tip of the driving pinion (tip sticking out of the nut) and continue it over onto the nut, across one side of the nut, and over onto the yoke. In the same plane as the first paint mark, make a reference line from the circular deflector, at the base of the yoke, and onto the face of the pinion housing casting. This will give you a reference when you go to put everything back together to line everything up.

A six-point socket (1-1/16", best I remember) will be less likely to round the points off the nut than a 12-point socket. Take your socket and paint a line on the outside of  it so that the paint line on the socket lines up with the paint mark on the side of the pinion nut --when you place the socket over the nut.

I made a tool approx 18" long to hold the yoke with to keep it from turning while loosening the nut with a 1/2" drive breakover bar. Before breaking the nut loose, write on a piece of paper where the clocking of the mark is. Example, let's say the initial reference is at 11:00 o'clock. Write down 11:00.

Place the socket/breakover bar on the nut (with the mark on the socket lined up with the mark on the nut) Start breaking the nut loose while counting the revolutions of the reference mark on the socket that it takes to remove the nut. On average for me, this has been about 16-17 revolutions but, your setup may vary.

When the nut comes free, write down the number of revolutions it took and what the clock position was when you could remove the nut from the threaded pinion stem. --this will be the oriention/starting point when you go to put the nut back on.

Remove the old seal and clean out the bore where the new seal will seat into. I put a thin layer of Ultra Black on the sealing surface of the new seals metal housing, prior to driving the new seal into the bore. Take some gear oil and smear it around the rubber lip of the seal, before you put the new seal over the pinion stem.

Once the new seal is installed, slide the yoke on, index the line on the pinion nut to the position it was in when it came off (reference your notes). Begin tightening the nut while counting the revolutions required to put it back to the intial clocking position when you started.

Once there, put your hand on the yoke and make sure that you can't wiggle it up/down or side to side. You may have to go a LITTLE tigher to eliminate any possible slack but if you have to go very far beyond the original revolution number, something is not right. If the crush collar is compressed more than it was to start with, the crush collar will have to be replaced and the driving pinion depth/preload resetup. :-(

My experience has been that it takes over 300 ft-lbs of torque to crush the spacer.   On my 9" rear, I removed the nut, replaced the seal and re-torqued the pinion nut to 150 ft-lbs or so.  As Ultrastang said, it's not a bad idea to mark the initial position of the nut for a sanity check.   I'd also check the pinion resistance both before and after to be sure nothing's changed.

One thing I do differently is to just remove the five bolts holding the bearing retainer in the housing and pull out the entire bearing and pinion shaft/gear assembly.  This part requires no set up as the depth is not changed by doing it if you put the same number of shims back in as what came out.  It also gives opportunity to replace the large o-ring around the bearing retainer.

That gets the assembly so you can work on it on your workbench rather than laying under the car.

I clamp the companion flange in my press to hold it still while I use a large socket to remove the big nut, after carefully marking the nut, flange and shaft so it can all be reassembled exactly as it came apart. Pull off the flange (yoke) and replace the seal.  Put it back together just like it came apart, lining up all the marks, then tighten to exactly where it was before, plus about 1/16" of extra travel from the original marks.  It will work fine.

These rear ends are not rocket science, especially since most of them have 100,000 miles on them.  If it worked OK prior to removal, it will work fine afterwards.

Once the assembly is torqued down as described clean the mating surface, install a new o-ring if needed, and bolt it back in place and torque down the five bolts.  If not using a new o-ring, I usually put a small amount of RTV around the area where the old one fits.

ALSO, do not forget to grease the ID of the new seal you installed!

To determine if you have a hunting or non-hunting, you need the tooth counts - this site shows tooth counts for 9" Fords -  http://www.oldmanengineering.com/9inch.htm  

From this you can determine the unique assembly phases for the gear set - NA -  This is the product of the prime factors common to each gear.  For a 3.70 gear with 10 and 37 teeth, the prime factors are 1,2, and 5 for the the 10-tooth pinon and 1 and 37 for the ring gear,  the common factors are 1, and NA is 1.  This is a hunting gear set and can be taken apart are reassembled without any marks.  The 3.50 gear in this posting has 10 and 35 teeth - 1, 2, and 5 and 1, 5, and 7 respectively.  NA is equal to 1 x 5, or 5.  This means that if the pinon is removed, there are 5 possible ways that it can be reinstalled.  When NA is greater than 1, certain teeth on the pinion will only match with specific teeth on the ring gear.  For example, pinion tooth 1 on the pinion will mesh with tooth 1 on the ring gear and tooth 1 on the pinion will never contact tooth 2 on the ring.  The gears may or may not be lapped when manufactured but they will certainly "lap" or wear a specific pattern in service.  If you take out the pinion and install it where tooth 1 now contacts tooth 2 of the ring gear, there is a potential for noise and vibration because you have teeth in contact that have never been in contact before.  The key word is potential - not every gear will make a racket when installed improperly but the chances are pretty high that it will be noisier.  So, with the 3.50 gear, which is a partial-non-hunting set, you need to be careful.  The 3.70 doesn't care, and a gear like a 3.00 set with 13 and 39 teeth has a NA of 13!  This is a non-hunting set.

The howling frequency you will hear with improper reassembly is called the gear assembly phase frequency - at least in the rotating machinery world.  This is equal to the RPM of the driveshaft/pinion times the number of pinion teeth divided by the product of NA and 60.  This gives you the frequency in Hz.  For a 3.50 gear set in 4th gear (1:1) at 3000 RPM, the GAPF is equal to 3000x10/5x60, or 100 Hz.  A gear assembly phase error also tends to produce a lot of harmonics or multiples of the GAPF which contributes to the "harshness" of the tone.

Good design practice says that both gears should be prime numbers, next best is a NA equal to 1, and not so good is NA greater than 1.  But, you also need to get a ratio to match the job.  As long as you pay attention to assembly, you will be fine.

A little trick I learned the hard way when installing a new pinion seal in any rear diff. assy. is to pack the spring under the lip of the pinion seal on the back side, with wheel bearing grease or chassis lube.  Sometimes with driving in the seal, the spring jumps out and it is crushed between the pinion yoke and the bearing.  You then have an instant driveline vibration.  Time to redo. Hope this saves someone some time.