My 97 Blazer's transmission slips on start up when initially put into gear, Reverse or drive, on start up. If I let the engine idle for about 30 sec it works fine. there doesn't seem to be any slipping while driving .
Thanx Skully69

 

Check fluid level while the engine is running, and the transmission is warm. If it's low add some fluid to get the level up. Also might want to try additive like lucas.