Sounds to me like the T/C isnt locked into the pump in the front of the trans properly. In order to fix this and the easiest way to fix it is, drop the trans and disconnect the T/C from the Flex disk on the engine. Then with the trans and T/C seperated from each other push the T/C onto the input shaft of the tranny and turn it, you should feel the T/C go into the tranny more and there should be a prositive clunking sound. Now when you spin the T/C it should take more effort because it is properly engauged with the tranny pump.
I hope that helps and i didnt confuse you.
1999 XJ Limited.... R.I.P 10/27/06
1998 XJ Classic, stock for now.