I had the transmission replaced about 8 months ago and the u-joints replaced about the same. Now at about 45mph, if I hold the speed and let off a little, it starts a bad vibration which seems to come from the rear of the truck. Hard to say. I believe it might be the carrier bearing. It has been sort of weird. It doesnt happen all the time. The tranny was built be Evan's in Conroe. Probably one of the top transmission shops in the south. Not the cheapest but he builds top quality. I went with the upgraded torque convertor and the shift kit. The tranny hits hard, but I dont really notice it anymore. Any other things to look for in the vibration. I used spicer joints from Napa.

 

The only thing that comes to mind is a problem a lot of people had with the tailshaft bushing.

Check out these replies and see if the symptoms apply to your situation.

Good luck.