Between 40-50 mph there is a annoying vibration. Below 40 and after 50 mph it disappears. When you let off the gas it goes away.
Things we have checked so far:
output shaft wear
front end alignment
front/rear u joints
all upper/lower ball joints.
Tires(all just changed and balanced)
Any suggestions? How about driveshaft balance?


Thanks mike

At this point, I would take a good look at the transmission, and motor mounts for the rubber showing signs of breaking down.

When you check the u joints, did you remove the driveshaft and check the smoothness of the bearing caps, or were you just looking for missing/rusty needle bearings? I've seen a lot of the needle bearings cause step marks on the cross shaft that didn't have missing bearings or show rust, yet. If you drop out the driveshaft and any of the u joints don't turn smoothly and freely, replace that joint.


Have you checked the condition of your front wheel bearings?

Have you checked to be sure the pinion nut on the rear end has not backed off a bit? Gene