Chevrolet Repair: 86 chevy truck drivetrain vibration, wheel bearings, chevy truck

Question
I have a BAD vibration in my truck from 25 to 40 mph. not every time i drive but most. replaced wheel bearings, tires, and it's still there.     checked universals......good, however there is a LITTLE wobble at the tailshaft and nose of the driveshaft yoke.    the flywheel to converter bolts are also tight............any advice would be greatly apprreachated.                     truck is 86 c-10 with a 305 auto tranny single driveshaft

Answer
Hello Albert,
If you haven't already done so, take the truck to about 40 or so, and get it to vibrate, then put it in neutral, and let off the gas to see if it still vibrates from rolling, or if it goes away when the engine is not connected.
I am thinking belt driven accessory, like A/C compressor maybe?
Somewhere in there I would be revving the engine to about the same RPM's, but I'd bet you have already tried that.
If it remains while coasting at speed, I would start by jacking up the rear wheels, and operate it at about that speed, and watch the driveshaft.
Follow that by using the spare, and switching tires one at a time, and see if it is a wheel/tire related problem.
Van