These older transmissions use "soft yellow metal parts" and some gear oil will not play nice with them. If you're using a std type gear oil, make sure it is GL-4 or GL-5 compliant. If you're using a synthetic, make sure it says it's "synchro safe" for transmissions with "soft yellow metal parts" as the synchros are particularly vulnerable to the effects of some GL-5 synthetics.

Read this thread for more information:

http://www.toyotavantech.com/forum/s...ch-Replacement

If it's not a gear oil induced problem, then it's likely a mistake made somewhere in the mechanical work you had done. Good luck on getting it sorted out. Tim