Jeep Cherokee smells when I back up

I have a 2000 Jeep Cherokee with about 175,000 miles on it. In the past three weeks, I’ve noticed that after I’ve driven for a while and if I stop and back up, say to parallel park, I smell an acrid odor, like burning rubber or oil.

I don’t smell this if I’m driving into the garage and close the door. I don’t smell this when I’m just backing out of the garage in the morning and the car is cold.

I only smell it after I’ve driven for a while and when I put the car in reverse.

I’ve checked all the fluid levels and everything is OK. The gauges show everything is OK. The car isn’t running hot, etc.

Any idea what this would be?

Have you checked the rear differential fluid level?