Change your AT fluid. Get OEM fluid (I think it's drexron III or however you spell dexron)
You need 2 new gaskets and a new filter while your at it. You CAN do this on some tall jack stands but it will be a pain in the ass. Find a lift.
Crack open your differential and inspect the gears as best you can. Best would to be to completely remove it and take it apart and examine the innermost gears, or the output shaft (to the wheels) gears as those are the ones that "turn" when your back wheels are spinning at different RPM's (IE: turning the car) and thus, would skip around.
Once you find out the problem, (diff or trans, I'm betting heavily it's the diff since you say it only happens when you turn, which is cheep compared to the transmission) then you can decide wither to upgrade, replace, or just live with it until it catastrophically dies and then replace the entire unit.
If you can describe it in more detail and give us conditions of when this happens, maybe some of us can narrow it down further than "your transmission and or your differential is screwed up."