When I put my truck in 4x4 mode, I hear some horrible grinding noises.The 4x4 still works, but the noise has me worried. So, I disconnected my front driveshaft and checks the u-joints. The drivers side u-joint on the axle is siezed which could have caused all my problems. When I rotate the front diff by hand I can hear a grinding noise inside of the cover. I know it is not the hubs because it makes the noise when I hold each axle still. Shouldnt my front differential be completely silent? When I rotate my transfercase, it is smooth with no noise at all.

Background info:
I have manual hubs. The noise started when the hubs were unlocked, and the truck was in 4x4 mode. So that seized axle was still in motion. I was also on the highway at the time the noise started. I checked the diff fluid and it was very low. I filled it up and the noise persisted. 4x4 still works, but the noise is scary.

 

bump

 

Year? model? mods?

 

Sorry. Good question lol. 1997 Ford Ranger XLT 4x4. No mods.
Is it safe to assume that my front diff should not make any noise when rotating it by hand?
Also, does my whole driver's side axle need to be replaced, or can the u-joint on the axle be swapped?

 

These are simple questions aren't they?

 

Most likely broken spider gears in the diff.