my freind just baught a 98 silverado ext. cab short bed 4x4 with a 6 inch suspension lift and 35 inch tires.
when he accelerates hard the hole truck vibrates up and down, pretty bad but not violent, but still verry noticable.
what could be the problem to this?

 

check ball joints.....

 

okay, vibration. these kinds of questions are tricky, but i know its suspension related since you said 6 inch lift. how does it vibrate? you said it goes up and down, that usually means something rotating is out of balance. I get a lot of stories about lifted vehicles and terrible vibrations and wobble and the death wobble. usually those issues are dealing with poorly aligned steering equipment up front like your idler arm, pitman and center link. now, vibrating up and down movement... first thing to do is go under the truck, and shake everything to check for looseness. start up front, with the steering linkage. shake the center link to see if theres any play, check the ball joints for play, then move on back an check the trans mount, check the driveshaft for looseness. the general rule of thumb, if there is any play in your steering linkage, replace it. if it jiggles some at the joints, try tightening the nut, but thats almost never the case. if you jack the truck up, and check the ball joint for play, remember this; when the bearings are cold, and properly set, you will get a very small bit of play in the tire. this is normal. dont confuse that with ball joints. if you feel any play at all in the joints, replace. let us know what ya find.

 

Does his truck have autotrac?

 

I think what you may have going on is that the rear drive shaft is a little short, mine did that after I lifted it and I needed to have a longer rear drive shaft.

 

Sounds like a driveshaft issue for sure, check slip yoke depth in the tranny, check u-joints and if everything seems good then roll the driveshaft on flat ground and see if someone or something has bent it.

 

There is a drive shaft alignment kit available, and that may be your solution. The on that I have is from Beltech.

Gentlemen.. correct me on this please. Would a one or two piece drive shaft have any bearing with this issue?

 

i would say U-Joints, improper drive shaft angle. ether a one piece or two piece if the shaft or PINION angle is off, will give you problems. there are kits available to correct it. one should be for lowering the trans mount, and the same for lowering the center section of the 2 piece drive shaft..they should of come with the lift kit..

good luck