Have you eliminated your u-joints/double-cardan or slip yoke as the source of the problem?

When the u-joint was gone on the pinion end of the driveshaft of my old truck, it felt nice and tight while mounted on the truck but made some awful noises (which I thought was the ring and pinion). I pulled the driveshaft off and noticed the u-joint had fallen apart - caps were cracked, needle bearings were gone.

Give everything a good inspection, then a greasing, and pull the DS if you have to.

It's natural for the gear oil to turn dark, remember your rear 3rd is always working but your t-case isn't.