So I did a complete suspension/steering/wheel bearing overhaul on my 91 pickup, 22re 4x4. Went with the Old Man Emu front and rear, medium load CS019R in the rear with corresponding shocks and new bushings. Before I did this there was a loud somewhat high pitched whining sound that from what I could tell was coming from the rear end, became noticeable around 25 MPH and got worse as speed increased. I was hoping it was a wheel bearing but after the job was done the whining sounds the same but now I get a lower pitched "humming" type sound that comes and goes and changes pitch at various speeds and is near unbearable at freeway speeds. I thought I read in an older thread that lifting the rear changes the angle of the drive shaft and can cause noise, but I cant seem to find that thread right now. Just for kicks I replaced the drive shaft u joints (truck has 430,000 miles so I'm gradually replacing mostly everything) but didn't make any difference, I have the 2 joint type if that matters. Is it true about the driveshaft angle causing this, and how can I correct this? Also, about the high pitched whining, since I've ruled out wheel bearings and u joints at this point can I assume it's a rear diff issue? I've tried putting the transmission in neutral while at freeway speed and it sounds the same so would it be safe to rule out the transmission? And when I first bought the truck it was low on gear oil in the diff so I drained and replaced it, what came out looked pretty grey/murky so I'm sure that's not a good sign

 

One thing you can do to try and isolate the noise is to remove the rear drive shaft and drive around in 4wd. If it is a rear wheel bearing you'll still hear the noise, and if it is transmission, you'll still hear it. If it is u-joint, transfer case output, or rear diff pinion, you won't hear the noise any more. With the shaft out, is there any in/out play in the pinion flange on the rear diff?

 

Sucks, but lessons learned only modify things when everything is working.

You didn’t mention size of lift.

4crawler.com has loads of info. On his site there is info on driveline, types of joints, and setup requirements for each type joint. The reason i’m not telling answers is best if you’re gonna modify things on your truck it’s best to understand how things work. Understanding concepts will save time and money.

our rear axle is a semi-floater design. This means if a rear outer axle bearing fails the axle shaft will separate from the housing. Not a situation you wanna drive on.

Check pinion for play. In-out, side to side, how far does it turn before tires turn. Check t-case output flange for play also. Honestly probably best to pull rear axle apart and inspect all. Play in outer axle bearings. Gears, debris in housing.

 

The OME application guide said 1.75" lift. I won't have time to do much investigating till this weekend, unfortunately. I'll try driving without the rear drive shaft then and see how it goes