I've had more than my share of rear seal problems with my engine. Not too long ago there was a post from a member who had the rope seal replaced while the engine was in the car. But, My opinion is to pull the engine to replace the seal.
I just did an R&R of my 373 posi diff. After rolling under the car on a creeper for a few hours, I wish I had something like that ramp to give myself some more room. Still would have had to get the rear wheels in the air but could be done on the ramp somehow. Wouldn't want to have to move it though. Looks like it's in bolt together sections?