I have a 1997 Cherokee Sport 4.0 inline 6 that I had to replace the backing plates on, (the things that the brakes, pins and springs mount on mount on).
I had to take both axles out of the rear end to get to the backing plates because they wouldn't fit over the wheel hubs. The axles are the type with the C clip and pin that goes through the spider Gears. After putting everything back together in reverse order from taking it apart the rear and spins free while in park or in gear. All spider gears are turning and meshing with each other correctly, while in park or gear the ring and pinion won't turn but the axles turn free, any ideas??

 

Perfectly normal. The transmision Parking Pall keeps the rear output shaft from tutning, which means the nothing connected to the transmission aft end can rotate. Once the tires are on the ground they can't turn either. Watch the following to get a better understanding of how your differential works, you'll understand why your spiders are rotating around a non-rotating ring gear.

 

Awe some video, I thought I broke something LOL, thanks for the schooling