I have a 98 XJ, mostly stock. I was just curious, so I pointed an IR thermometer at my differentials. After about a 10 mile drive (in 2WD) the rear reads 108 degrees, while the front is at 118. Seems odd to me considering the rear is doing all the work. Can anybody explain why this is? Is this normal?

It's possible I have different oil in the front, I've changed the rear with non synthetic 85w140, I'm not sure what's in the front, have not changed it yet. But I would also think the thicker oil would run hotter...

Not really a problem, I'm just wondering why. I'll have to check them next time I go on a longer drive.

 

Try changing the differential oil and then recheck the temp.

 

The front diff has a heater that has a normal operating temp of 210 degrees above it, the rear diff does not.

 

Yep, nope.
The rear differential could only "be doing all the work" if it was 2wd.