Hum/slight vibration above 60mph '95 4Runner 4x4

Ok. I can’t figure this out, and 2 mechanics I took it to haven’t figured it out yet. When I am at highway speeds, 6-70 mph, and I gently let off the gas, I can feel a slight vibration in the floor. You can also hear it if you listen. It stops when I completely let off the gas though, unless I’m going downhill, then it hums a few seconds longer before going away. I’ve had an alignment done, brand new pitman arm, idler arm, steering damper, and tune up. Also repacked the bearings in the right front wheel.

The mechanics do not see anything else that looks worn. They said the u-joints look ok. I don’t get any clunking or banging anywhere. What else should I have them look at? Oh. And a tire guy said it could be some feathering on the front tire. Would that only cause the hum/vibration at highway speed? I was going to have the tires rotated and rebalanced to see if that fixes it.

I’m planning a 800 mile round trip camping trip in a week and would like to get this fixed before then,

I had a similar problem on my 4wd Ford truck, and it was the drive shaft u-joints. So about all I can think of it to make sure the drive shaft spline joint and u-joints have no play and are relubed with fresh grease. And make sure the rear differential is properly filled with the correct kind of gear oil for you vehicle. If this happened after a u-joint was replaced, or the drive shaft was removed, they might not have repositioned the parts correctly. At least on my Ford truck you are supposed to mark the drive shaft components before taking them apart, as they can go together more than one way, but there is only one way that is properly balanced.

This sounds exactly like U joints at the early stages of failure. The condition might be enough to cause a vibration, but not enough for a mechanic to detect. As time goes on, it will become worse and then the mechanics will recognize it.

Everything keeps leading me back to the U Joints. I think I might just have them replaced. The truck does have 180,000 miles on it.