Had that same problem when I did mine. I think it is because you jacked the back wheels and that pushes the axle forward. The axle pivots on the spring mount because of the torque tube. I could be wrong but nonetheless what I found works much better is to simply remove the crossmember. 4 bolts each side and its off in minutes. Of course you have to disconnect the rear trans mount from the cross member. I did it once when I changed the clutch and years later when the seal in the torque tube went.

1949 Chevy 1/2-Ton
"Sedgewick"
In the Gallery
1989 Caprice