FWIW, we had oil spots on the back of our van when i was a bit younger and it was the tranny leaking, it eventually gave out in SC (we were on our way back to PA). you might want to check the seals on the tranny. the pan might be leaking a bit, collecting dirt from the road and ending up on your rear end.

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-Trevor

OBK #12
1980 300SD 333,XXX miles - Totaled
1986 Mazda RX-7 212,XXX miles - impounded and auctioned off
2005 Jeep Liberty CRD Limited 33,000- SEGR, Provent, Fumoto