Transmission just rebuilt, rear wheels still not spinning...

I went to AAMCO a few days ago to see about my lack of AWD. They were certain that the transmission needed to be rebuilt, so they got in touch with my warranty company to pay for the rebuild (which picked up much of it). But anyway, since it's snowed here yesterday I got to drive it around and even though it shifts so much more smoothly now, the rear wheels still do not turn! On a slight icy incline the fronts would rev up but as I watched the rears from outside the door they would not do anything at all. What did the rebuilder miss?