This is regarding the little fiber washers in the carrier and between the axle shaft ends. How do I know if I need one, two, or one plus another shaved down?
There is no set this is how direction. If you can't spin the axles when the two half's are bolted together then it to tight, if you have a large amount of back and fourth slop then it too loose.
I generally use a brass washer instead of the little fiber thingie. The fiber does not last long. Brass lasts a long time and works great. Use a dial indicator on the end of one axle to determine the thickness needed during a rebuild.
I've never found evidence of one upon disassembly of a used T rear end. They just disintegrate.
You can tell it is gone if the axle shaft has in / out play with the wheel removed on the passenger side.
With the oil running around inside the axle, they get lots of lube. They do wear over time, as in 80 years, the top original disc has some wear on surface, you can see the 'dimple' left by the centering divot on the axle shaft!
When I took mine apart the fiber spacer was as described earlier, obviously compressed, with the edge and the center still the original thickness and otherwise compressed by the ends of the axles. I don't know how long it had been in service as the rearend had been apart before. One thrust washer had been replaced with brass while one was the original babbit.