is something wrong with my rear end
If you don't know when the fluid was last changed, or if posi additive was put in if it was changed?
Why not do a Fluid change and put in some proper fluid and get the additive while you're at it.
Of course many people will have their own ideas about this, but really, it's your first "way to go" on seeing if the differential needs a better "wash" in it.
Then drive it till it gives up.

