Do it yourself.

How hard could it be ?

I tackled a TH400 auto in my old suburban in my garage with NO prior experience and came out fine.
I bought one of those scissor transmission jacks from Northern Tool for about $60 and did it ALONE with no help at all.

The key is getting the car up on stands high enough to allow you to slide the old one out and the new one in.

( O, and it helps to have adequate tools and proper fluid on hand before you start )