when backing up my 1999 f150 4x4 it makes an awful squeak noise. at low speeds its more of a low pitch squeak and the faster I go the high pitch it gets. in a way it sounds as if something is grinding. again it's only in reverse. and it runs perfectly fine and no clunk so I doubt it's the u joints.

 

My truck did the exact thing and my u joints started popping about a week after it started, I changed them u joints and no noise since. If you can get someone to drive the truck just bend over and try to figure out where it's coming from.

 

It's your U-joints. Most likely the one on the diff side. I had the same thing happen to me. They felt good, no movement but squeaked in reverse. I pulled the drive shaft and could tell then they were in bad shape. If I can give you a tip it would be to take the driveshaft to a shop that specializes in them to have them replaced. Mine were a nightmare and I had the use of my brothers shop press to do the job. After 3 hours of swearing I ended up taking it to Pats Driveline here in town. They replaced the U-joints, the knuckle that bolts to the diff, checked the balance and painted it.

 

it's been doing it about a month now and still no other sounds indicating bad u joints

 

Have you been able to indicate where it's coming from? It's quick and easy to pull the drive shaft that way you will be able to move them to listen for sounds.

 

The ONLY way to properly diagnose a driveshaft is to pull it. Mine was doing the same thing as these guys have said. Doing the wiggle test didn't prove anything.... but when I pulled it, it became obvious that the joint by the diff was shot. It was full of rust in the cup.