RE: 5.9 Rear Main Seal
Sending unit is on the back of the block on top, where the bellhousing meets the block.
The rear main seal itself is cheap. Labor is the expensive part.
On an auto it would be a little easier though, just drop the tranny and flex plate and replace it that way.
No idea on shop labor, I'd guess they'll wanna charge around 6 or 7 hours labor as a rough guess.