Truck Repair: 89 Chevy S10 2wheel drive, chevy s10, s10 pickup

Question
I was wondering if you could help me locate the possible cause of my 89 S10 pickup.  It's 2.8L V6 and only 2 wheel drive.  It starts to shake when you get to about 60K and above, below that speed it runs perfect.  The whole truck shakes, but not the steering wheel.  I had the back checked, so they checked the tires, the breaks, the axle, the rotors ect... and they found nothing.  It's weird because the steering wheel does not shake, but the whole cabin shakes (ie. dashboard, seats, floors, and box (i have a canopy on the box and it shakes too).
Any help would be appreciated.
Thanks


Answer
First, get it going over 60, where it is shaking, and carefully put it in neutral, and let off the gas.
If the shake quits, it could be engine, or fan belt caused.

If the shake continues, it may be a bent driveshaft, or bad "U"joints. Put the rear on jack stands, or up on a hoist, and operate it in gear at about the same speed.
Look at the driveshaft.

Van