I noticed like a slight transmission slip when I left for work this morning. Once I got up to highway speeds things were fine and I made a mental note to check the fluid. 15 minutes later I pulled in a gas station to get some coffee. When I tried to leave, the car wouldn't go into reverse.

I gave it a bit more gas and it lurched backwards in a real ugly way, I put it back in drive and it moved forward. I tried to get it into neutral but it just acts like it's in park. It will move in drive but that's it. I had it towed out of the gas station parking spot to my workplace where I'm trying to figure out what to do with it.

I talked to a couple diesel truck mechanics and they said probably to just take it to a transmission place. I'm more or less planning to do that but then I remembered that my rear main seal is leaking (seeping).

Would I be better off taking it to a place that could do this at once (like a volvo dealer)?

The car has 123,000 on it and we are planning on keeping it for at least another several years. Any opinions are sure appreciated.