My clutch did that when I had an OEM-type on and was at the drag strip, I ended up compressing all the springs in the hub to the point that they allowed the clutch to chatter constantly, and it gradually went downhill from there. It did last another 5 months after that severe slipping though on the street.

Also, a leaking rear main seal won't affect the clutch too much, but a leaking shaft seal on the transaxle definately will.