[Mark Yacavone]

Good job on the car, Dave.
I would attempt to fix it before going to Fontana.
Did you have the tires spun balanced? 0-0, while you were standing there?
Who built the driveshaft? .083 wall, 3" ? Should be fine in this application.
Did you bottom it out, then pull back about an inch , for installation?

You have adjustable upper bars? Did you check U-joint angles with a protractor?
This is probably the biggie here.
I don't believe it's the transmission. I check the back half drag many times, by spinning it with a yoke in place .Bent out put would have shown up then.
No extra roller bearings in this trans. It is your basic, entry level model.