My 2000 Jeep Cherokee makes a humming noise at those speeds. It is only when I push on the gas to maintain speed, no when I accelerate or slow down. Any ideas?

 

you might have an alignment issue. or possible some wheel bearings wearing out.

 

Quite possibly a pinion brg in the rear axle

 

I agree. I had that problem on my ZJ. The noise would go away when you released the throttle, and come back with any throttle pressure. It started about 30-35 MPH.

 

Pinion bearing sounds realistic. As far as a repair, I am assuming that this would be a PITA?

 

Disconnect your u-joint from the rear yoke and then see if there is any play when trying to push or pull the yoke forward and back. If there is some movement you can try to tighten the yoke retaining nut just enough to eliminate the play. DO NOT JUST TIGHTEN THE NUT ALL THE WAY OR ELSE YOU MAY DAMAGE THE INTERNAL PARTS. Only enough to eliminate the forward & back movement. I just did this to mine yesterday while changing the pinon seal and and installing a sleeve over the yoke seal surface. Just got back today from a 100 mile trip with no leaks or humming.

 

Sometimes when I slow down using my breaks from a higher speed right around 20-25mph sometimes I get a small clunk forward....could tightening the nut also eliminate this as well? Great Idea, thanks!

 

Agreed ! Mine had the same problem. Noise at 30 and 60 mph. I inspected the driveshaft and replaced the ujoints, and tightened the yke nut in the process, not sure which one solved the problem, but I'd inspect the joints, and if they look solid, the yoke is the culprit!

 

That could be many different things. Some things I'd inspect are possible loose u-bolts at the rear axle, bad shocks, bad motor or trans mounts, loose exhaust, bad u-joints? Check for leaks and play at the pinon seal as I indicated before.