you can do it yourself. it isn't a difficult job, just kinda messy and a bit time consuming. you don't need to drop the tranny either.
for starters, are you absolutely certain that it's the rear main that's leaking? a leaky valve cover can cause oil to run down the back of the motor and seem just like a rms leak. take a clean rag and wipe along the back of the motor and valve cover just to be sure.
you can buy the seal at most local parts stores. got mine from napa for under $20 i think. no special tools really.. some sockets, floor jack, a piece 2x4, a mallet or dead blow hammer, a punch (a stick welding rod/electrode worked best for me when tapping out the old upper seal), and that's about it.
here's a great step by step write-up with pics that'll tell you exactly what you need to do.
Rear Main Seal Service and Replacement