I have been fighting that for many years.  Finally, after buying a digital level that measures to tenth of a degree and installing a TKO with custom tranny hump and crossmember to get the right angle, I have a tranny that poimts down at 2.9 degrees and a rear end that point up at 2.5 degrees.  The car is alot smoother but I still think there is a little something going on.  It just isn't perfectly smooth like my 66 truck was.
Your ride height sounds to be same as mine,so should be similar in that regard, but ride height can have alot to do with it, I think.  Even though the ANGLES may be the same, the vertical offset between the two ujoints can affect it.  I think they call it included angle of the two, but read up on some of the ujoint sites. They have good info.  Figure like this:  If the tranny points down 3 degree and the rear points up three degrees, but the rear ujoint is HIGHER than the front one, that can cause the angles to be greater than three degrees at each ujoint connection.  THAT will cause problems.  Mine happen to be right at equal height, but still not smooth as I would like.  I wish there was a magic pill because I think I have thrown about $10k at the problem and stillperceive to have it, but less of it than before.

Note: Versailles rear end pinion angle is NOT parallel to the angle of the spring perches like normal Mustang rear ends.  I have three degree shims in mine to get it closer to a normal rear.  I am thinking next winter, if this is still a bother on the bash trip, I will invest in a new 9" housing like the one Hornman bought.  Mine came from a Versailles that was rearended, so always in the back of my mind is the question of whether it may be bent.

Something else to try. I had the same problem, but at a slightly higher speed after going from a c4 to a t5. Like me, you probably had to have your driveshaft shortened. Most shops only balance the ds to around 2000 rpm. Jack up the rear of the car and support securely with jack stands and chocks on the front wheels. Get a couple of 3" hose clamps and put one on the driveshaft and tighten. Start the car, put it in fourth and run it up to about 60 mph. Note the amount of vibration. Shut off the car and make a mark on the driveshaft where the screw is on the clamp. Loosen clamp and rotate it 90 degrees on the shaft and re-tighten. Spin the rear up again and note the amount of vibration. Repeat steps as needed. If at some point you notice a decrease in vibration, that indicates an out of balance driveshaft. Since your vibration is occuring at a lower speed than mine was, you may need to try the second clamp, matching the screw placement with the first. If this helps, you can have the ds rebalanced (good luck finding a shop that will do it correctly) or just leave the hose clamps on and drive the thing (my solution). If it doesn't help, take off the clamps and never admit to anyone you ever tried it. After 7 years mine is still smooth on the interstate and over 105 mph at the drag strip.

at wrote:

Something else to try. I had the same problem, but at a slightly higher speed after going from a c4 to a t5. Like me, you probably had to have your driveshaft shortened. Most shops only balance the ds to around 2000 rpm. Jack up the rear of the car and support securely with jack stands and chocks on the front wheels. Get a couple of 3" hose clamps and put one on the driveshaft and tighten. Start the car, put it in fourth and run it up to about 60 mph. Note the amount of vibration. Shut off the car and make a mark on the driveshaft where the screw is on the clamp. Loosen clamp and rotate it 90 degrees on the shaft and re-tighten. Spin the rear up again and note the amount of vibration. Repeat steps as needed. If at some point you notice a decrease in vibration, that indicates an out of balance driveshaft. Since your vibration is occuring at a lower speed than mine was, you may need to try the second clamp, matching the screw placement with the first. If this helps, you can have the ds rebalanced (good luck finding a shop that will do it correctly) or just leave the hose clamps on and drive the thing (my solution). If it doesn't help, take off the clamps and never admit to anyone you ever tried it. After 7 years mine is still smooth on the interstate and over 105 mph at the drag strip.

   Where on the Driveshaft do you install the 3" clamp?    Towards the transmission or more towards the axle?  Thanks Steve69
 

at wrote:

Steve69 wrote:

   Where on the Driveshaft do you install the 3" clamp?    Towards the transmission or more towards the axle?  Thanks Steve69
 

I put it on toward the rear. I didn't like the front because of the proximity of the parking brake cables. Fine tuning of balance can be affected by the fore/aft placement. In my case the rear worked out fine. Also the clamp should be installed with the loose end facing against the ds rotation. That way if anything should get kicked up from the road, it won't get caught under the end.

I saw BillyC's pic of his missing weight but he never let us know the outcome. (hint,hint)

I had the drive shaft shortened and balanced about 4 years ago at a shop in Virginia Beach, Va. I took it back to him thinking I would gladly pay to have it re-balanced. He is doing it no charge. It's in the shop right now. I'll be on the road this weekend.

Here is a great write-up on why the solution (if things are properly balanced) is to have exactly offsetting angles.

http://dazecars.com/dazed/JaguarIRS5.html