Rear crossmember removal

Though you don't mention it, I assume you've removed the rear spring.

Unless you're willing to cut big holes in the floor to reach those bolts from inside the car (not recommended), the rear crossmember has to come out. It is held in by one bolt at each end of the crossmember that goes up through a large round rubber bushing that fits in a sombrero-shaped stamping on the frame. The bushing tends to get "welded" into position over time, so it may take quite a bit of persuading. Leave the outer cover bolts in place hand-tight. Find a long crowbar and pry between the crossmember and the frame (strategically-placed pieces of wood might minimize cross member damage). Put something close beneath the differential to catch it so it doesn't crash to the concrete.

Two tricks that will help you that Wayne didn't mention.

Either place a floor jack under the differential to catch it (he implied this, but the 2-1/2 ton jacks usually catch better than jack stands, blocks, etc. but it must be about 2 inches below so that you can get the crossmember off.) or Remove the two bolts from the end of the crossmember and replace with ones about 4 inches long. Leave about 2 inches between the crossmember and the head of the bolt to catch it.

And when he said crowbar, he didn't mean one of those puny foot long nail pullers, he means a real one. The longer the better. Three foot is a start, and if you can get one 6 foot, use it.