Check out your u-joints.  Often they clunk when they are worn.  My first rear end lasted over 300,000 miles before I had to replace it.

lowercasesteve wrote:

Check out your u-joints.  Often they clunk when they are worn.  My first rear end lasted over 300,000 miles before I had to replace it.

X2, this is the first place I'd check.

Regarding the rear gears, in a typical automotive application the gears will last almost indefinitely.  What kills gears is heavy loads and sustained high speeds that heats up the gear lube and slowly draws the austemper out of the steel making the gears soft and prone to stress risers.  I would typically only worry about gear life in a truck used to tow or haul heavy loads.  Its harder to check in an 8" or 9" Ford rear, but you can check your backlash and see how much its opened up.  A wider backlash than spec however is typically not a sign of worn gears; its a sign of worn bearings allowing the pinion to move forward and the backlash to open.  Replacing the bearings and setting the gears back up will prevent a future failure.  When setting up used gears you really only need to pay attention to the coast side of the gears as far as pattern goes.  Just replacing bearings the setup should be very close, and some will say there's no need to check the pattern.  They'll also tell you the newly rebuilt rear always used to howl on deceleration, and be wrong on both counts. 
 

Alessandro wrote:

Hi , which rearend have you. ? If you have on the 8 inch like mine (oem) i can have a lot of info for you.
On my 8 inch i put new pinion , crown and a new limited slip diff ( powertrax , grip pro ).
Beware on this one ! I worked a lot to fit and what i have grind is one way only .
I had one big issue with this so. ,. Depends How is your ride i suggest to think about a 9 inch .

As is a long story , tell me something i got pics and info to share .
Ciao !

I like you Al !!!!!
We lose a lot in the "translation" butt  I still appreciate your input.
Hang in there with your Mustang and keep the suggestions/pictures coming!
6sally6

Hi , gmk24  
my rearend was tired and leaking but no problem with gears or strange whistle .
When i bought the car i was soon interested in t5 transmission , as mine was 4 manual .
Upgrading the rear end was a something to take note also because i need different gear ratio ( on axle) due to different t5 trans gear settings.
anyway i had to change pinion and crowd so i decided to look for differential , listen to suggestion and dig the web for info.
the power grip pro  i choose is intended for street use as limited diff slip means that manage the power from left and right without loosing traction if one goes on slippery .
This is mechanical working , with only gears , no friction inside that some others have 
( need to be replaced sometimes ). this was my choice from some like heaton and other that now i dont remenber.
Price was average and application seems to be right but is not like that - ( they say is for 8 And 9 inch ) but in mine wont fit without grinding the inside .
some pics will help 

So this is the hear that keep the bearing that touch the new differential , Seems that is only shiny but i have removed a lot , ( i dont suggest to anyone to do it )  the result in case of braking could be a big bang 
But in a way or  another i want this in so o make some other spacers to gain some .
The problem of touching is only the first because this thing dont allowes you to register the crown from left to right  . i mean that if you dont clear space you will not able to set up the backleash in  no way


the touching part  . I have grind also the black case of diff  
this pics is very clear  
LOOK at threads and bearing seats  . This part should be able to move , adjusting from right to left , but as you can see , is touching  and this dont give you any movement ( or better to say you can go only from this point to left ) 


on the other side you see the right position to begin , both they got to have moving 
This spacers bronze leaf as i remember 0.5 or 0.75 mm give me some more room without grinding anymore.inside pinion i choose for solid spacer , spend a lot of time to find the right shims to use ,  put on and take out 1000 times because the backleash is a game .
The pinion  move inside outside  and crowd move left right  so the print they leave on the tooth ( both ) needs to be read with prussian blue or contact paste .
incorrect patterns will broke in few or give bad wistle or become hot  .
look at one tooth at time you will see in the front the light blue spot more ore less like a oval  quite in the center , the rest is blue This is the pattern,
this procedure is done with centesimal comparator ( , in mm for me  ) and magnetic base 
When you have done all this with time and patience maybe  all will be fine  ( yes i sayd maybe , because grinding has make that little part weak than before ) 
Now i cant say if my job will give me problem or not  because car is not finished and little ride around my house  dont give me much as feedback . I can give you some feedback in a couple of month when will be road ready 

after all that i can say that it was better find a 9 inch and make over there all new instead of the little power  keeper  8 inch ,  if you think to rise the power like me .
Unfortunately being outside usa make a 9 inch a rare find so for that i choose to rebuild the old one . 
If one day i will find a 9 inch i will not think twice also if this means more job to do !  

If i were you i will look for eaton or anyway look at top notch brand so give you more and tested .

Hope this helps ! 
tell me something if you need 
bye