Do you know the last time the owner changed them? If not, I'd change the transmission, transfer case, and both diff fluids, just to be safe and start over with a fresh maintenance schedule.
I'd also change steering fluid and brake fluid, but that's more involved. Maybe antifreeze as well?
2000 TJ, 4.0L, 4" suspension lift, 35s, rear D44 and front HP30 both with ARBs. AX15, 4:1 T-case.