Can anyone tell me why

 

I dont have specifics on this, but a good look at your u-joints would be a place to start.
Under normal driving is there a clunk when you let off the gas or accelerate? How many miles on your rig?
Wayne

 

160000+ miles

 

The symptoms I see with worn u-joints is a high frequency vibration you feel in the seat rather than the steering wheel. The vibration is higher frequency than when you have a tire out of balance and it comes on around 45mph. There is some difference whether you are coasting down a hill or climbing up a hill, but I can't remember if the vibration comes on stronger going up the hill or coasting down. You can sometimes detect the slop by climbing under the van and rocking the drive shaft around back at the pumpkin. It's always the rear joint that wears out first.