Since I've had my 82 G20 I've replaced the engine, trans, u joints, motor and trans mounts, front wheel bearings,tires, wheels, shocks, springs etc. Still when I'm cruising down the road I have a vibration whether i'm on the accelerator or not (worse when I'm on it) I've had the wheels balanced twice, alignment from a reputable shop. My next move I guess is to rebuild the rear end? Has anyone had this issue? Any ideas of anything I might be missing? Thanks in advance! CHEERS!

Welcome Hawg.
Worse during acceleration?
You said you replaced the transmission so can we assume the tail shaft bushing is new and fits the front driveshaft yoke well?
Driveshaft out of balance?
Is the van raised or lowered? Pinion angle is good?
Rear axle bearings could be worn out or have been improperly replaced in the past. Sometimes the so called "repair" bearings will vibrate.

The following would shake all the time.....
Cooling fan out of balance, bent or semi-engaged?
Bend or damaged engine pulleys? Bad belts?
Flywheel, torque converter bad, loose?

Aggravating problem for sure......

Yep! It's slightly worse "under load" or when accelerating. I replaced the THM375 with a 700R4 from Monster transmissions. I had to fabricate the crossmember to adapt to the 700 R4 but, its beefier and stronger than the original. The rear height is stock .. the front has 3" drop spindles. It has a new harmonic balancer installed on the new engine and the pullies run true. The motor and transmission mounts I used were the poly urethane type and may add a little more vibration but the vibration I'm concerned with is when the vans rolling between 55 and 75 mph after 80 mph it smoothes out.

Welcome to the site Roadhawg ..... it sounds like unbalanced drive shaft and or carrier bearing ...

Again welcome

No carrier bearing on this puppy! I'm gonna start with replacing the poly trans mount with a factory one and balancing the driveshaft. Just returned home from having the local transmission shop go for a ride with me and he said thats a good place to start. He doesnt think its in the rear end. Thanks for the responses!