The rear main seal on my F350 is leaking. It was just a little last oil change (probably 7,500 miles ago), but it looked much worse when I changed the oil today.

Since the transmission needs to be pulled to change the seal, I’m wondering if I should consider replacing the transmission as well. How long should the transmission last? Truck has 140,000 miles. This is a job I’ll be paying someone else to do.

It would seem to me as long as you're pulling it a rebuild is in order, that's provided the rest of the truck in in good running order, engine, transfer case, axles, etc.

I vote for not messing with the transmission if everything is otherwise OK with it. Yes, if it goes bad later, you have to pay the R&R labor again, which if 4wd is fairly considerable, but that's a mighty big "if." If you do choose to rebuild or exchange it, how do you know something won't go wrong with that? I would personally take that gamble.

Also, be sure it really is the rear main seal. Oil pan gasket leaks and even from somewhere else altogether can trick you.

It could be the upper oil pan gasket which is a fairly common oil leak source on the 6.7 PowerStroke. It also requires removing the transmission, plus a bunch of other parts.