thank you, the rotted piece is in that cross-member before the bumper. the rotted part is above the differential. is it only me having this issue/Waittaminute here. Are you referring to that large crossmember shown in the picture above that supports the forward mounting tabs for the upper control arms for the rear end? The one located just ahead of the upper spring pockets and the shock mounts??
So those two black lumps peaking out from behind the spring pockets would be the last set of cabin/body mounts???
In a typical g-body frame there are only two major crossmembers, the front one which supports the front suspension and engine mounts, and the rear, which does pretty much the same thing for the rear suspension.
If you are seeking THAT rear crossmember, by the time someone could visit a yard or vendor, source a donor frame from which that piece of steel could be harvested, get it paid for, wrapped up for transport, and shipped to you. The total cost might prove to be more than if you found a complete donor vehicle and harvested the entire frame.
That crossmember is totally structural, and heavily tied into the side rails. It is not just a simple piece of "U" channel that is used under the box to stiffen the box floor. It is also the last structural fixture before the rear bumper. After it there are a couple of sets of rear body mounts but nothing else.
It would all depend on where and what is damaged on the existing crossmember whether or not the immediate damage could be cut away and a replacement insert, made from the same thickness of material and shaped as needed, could be installed instead.
Not saying that the rear crossmember could not be removed and replaced, only that the cost to acquire that replacement plus the cost of the labor and the precision and accuracy demanded to get it located and burned in exactly where it is supposed to be equals $$$$$$$$$$$$. Most shops won't even look at frames any more. The insurance and liability cost attached to doing the work plus the potential for lawsuits if the vehicle is repaired and then gets in a wreck, even if not frame related mostly equal, sorry no can do.
JUst me here, thinking that, if it is that rear crossmember in the picture that you want to replace, were it me, I'd be on the hunt for another frame. Yes, it is a lot of work to remove all the bits and pieces off the dead frame and attach them to a replacement but it is still less of a hassle than invading the structural integrity of your current frame to the degree you seem to feel may be necessary. Just my personal opinion here.