While I was rebuilding the shift linkage assembly, the transmission selector rod joint was removed from the selector shaft. This was a good opportunity to replace that seal, which is typically the main source of transmission fluid leakage. Because I had dropped the transmission down to remove the driveshaft from the transmission, this was also a good time to replace the shift detent plate, which is on top of the transmission.
To remove the selector shaft seal, I fashioned a pick out of a small screwdriver, procured just for this purpose. It took some time to get the old seal out, being careful not to nick the wall of the transmission output. I inserted the new one on with a socket. No more leaks!