Mike,
Besides the fact that this obviously needs to be repaired, the question arises, how did it happen?
The crossmember is welded to the towers just inboard of the rear springs. If I understand your description, the only cause I can picture would be that the front differential mounts have failed and allowed the pinion end of the differential housing to rise up shaply and come into contact with the crossmember. Repeated contact like this would cause the flowered out appearance you describe.
Are the front differential mounts solid? Can you see the drive shaft well enough to see if it displays any metal-to-metal contact, it may be hitting the cruciform plate as well.
With this type of damage, I would take a hard look at the rear mounts (on the second crossmember where the shocks mount) also as they are probably carrying a lot more torque load that designed for.
I just looked at the 2 body off rebuilds I am working on and do not think the crossmember can be replaced without removing the body. It does appear possible to get to it from the top by cutting out the parcel shelf. However, I think you would have to be satisfied with a repair, not replacement. replace the crossmember this way, just patch it.
I am sure finding this damage was not the best news you ever got, but on the other hand, no one was hurt as could have been the case if the crossmember failed while driving.