While you're servicing the driveline ... if it's never been done, or if you don't know ... not a bad time to change the F&R diff fluid, as well as the transfer case fluid. Be aware that the transfer case calls for a different fluid than that F&R diffs ... they both use 75W90. The transfer case calls for a 75W. This was new to me .... dealer parts department looked at me like I was crazy. Said in the shop they use regular gear lube (75w90). Once I saw the difference in consistency of the two lubes I knew how wrong that was!
I found a lube on Amazon ... Ravenol J1C1003 SAE 75W. Was less than $20/per quart, and you'll need 2 quarts.