I have a 97 Roush F-150. I have done some mods to it, but the problem existed before I did these, and still exists.

At about 95 - 100 MPH the truck feels like I am running over washboard concrete. I saw postings from a similar problem on this board, but it was at a much lower speed.

Anybody got any advice?

 

Try getting your tires rebalanced for HIGH Speed. What's the difference between a Roush, and a stock F-150?

 

Sounds like you could have a problem with the driveshaft being out of balance. I have heard of driveshafts in lincolns vibrating so bad at high speeds(140 MPH) that they actually fall out! Lincoln had to replace these driveshafts with stainless steel the prevent this problem. You might look to see if there are any missing weights from you driveshaft or tires. Might also check the speed rating on your tires.

 

if it was the driveshaft you can tell by backing of a little, it won't shake near as bad without a load on it. if it vibrates under power or not it's probly the tires, get them balanced on the truck[ warning if you do that you have to do it every time you rotate but it's worth it] or you could just slow down

 

I would start by checking the simple stuff like tires, tie rods, ball joints, drive shaft, wheel bearings. Put the rear wheels SAFELY on jacks and see if you feel the same sensation. Be careful.

 

consider yourself lucky! I doubt any chevys go
fast enough to experience this problem. and after
all, it is a truck. Not a mid engine unibody sports car,,,I am joking by the way. wish I had the b##ls to drive mine that fast!