Can you find the vibration???

[Gmelahn]

I would like to start off saying that this is my first post on this forum but I have been here for a few months now. What a great site!!!

I have a 1972 K10 new motor(stock w/ rv cam), a 4" Rancho lift, rebuilt posi 3.73, turbo 350, np203. Truck sat for a while(was my dads) but has a rebuilt transmission. I also just had the driveline straightened and balanced, the rear-end rebuilt, the transfer case had the coupler and transfer case spline replaced and new tires 33x12.5x15 with less than 200 miles.

Problem:

I get a bad vibration at freeway speeds, the vibration is constant but is strong for a few seconds then it gets lighter. The vibration has a consistent tempo that lasts until I am under about 45-50 mph. There is also noticeable vibration(while not as strong) under some accelerations. Also when I let off the gas and then reapply the gas quickly there is a noticeable clunking coming from the transfer case/transmission. I can control the clunking by feathering on and off the throttle.

The engine was just checked out by our Mechanic today and he said the vibration is not engine related. He did however recommend new transmission and transfer case mounts, which are now on order.

I have searched as many different combinations of words to try and figure out what else the vibration could be, I have had a lot of good tips but I am sick of throwing money at this problem.

What should I look for, Torque converter? Fly wheel? Wheel bearings? PLEASE HELP!!!

Thanks,