The "brass thrust bearing" is only there for the roller needle cage, nothing else. If you add more than the original one, the outer race will be protruding from the housing.
I'm puzzled as to what you're trying to fix. Assembled the way they come from the factory, a properly shimmed and maintained rear drive will last 200k miles or more.
The stack height between the thrust washer and the seat of the flange cover. With out the gasket and .050 inch shim the height is .025 inch to tight. The cover will not seat against the housing.
Adding the two caskets and shim is .027 clearance.
This allows the crown gear move .027 inch away from the pinion gear. Creating slack or clearance between the pinion and crown gear. Using 4 gaskets is .030 or .005 inch clearance.
If the big bearing is installed fully seated in the cover and the crown gear fully seated in the big bearing inner race, the shaft sits .025 inch off the brass washer. Then if the pinion gear is adjusted, the pinion an crown mesh too tight if the shaft moves toward the brass washer. I checked to see if this happens by changing the shims in the pinion bearing housing. Moving the pinion gear in and out moved the crown gear and shaft . 025in. Toward or away from the washer or outer cover with two gaskets and .050 shim.
The Stock. 050 shim is not always the correct thickness, in my assembly.