IF you change axle types you need the bearing case that goes with it.

FOUR part numbers will come up in the dealer's EPC when he looks up the axle for your T100. These are, axle, bearing case, oil seal and bearing.

In case you still want to consider used parts:

The NEW axle is found in 03/98 to END T100. Also 02/99 to 08/02 Tundra with model UCK30 UCK40 VCK30 VCK40.

The bearing case for the NEW axle on the T100 is found on 09/00 to 09/04 TACOMA with model RZN140 RZN150 VZN150. But, you probably can't get just this at a junkyard. One from a bent axle will be fine tho!

I agree with Erik 100% about not replacing an axle that is not bent. The FSM provides a runout spec for the shaft. This is easy to check with the right equipment. Even though I dont know why bent axles seem to be so common, this is 1" thick steel shaft. It isnt going to bend without a good reason.