What did I break?

I had the same thing happen with the axles that Yukon used to sell as "a body 8 3/4 large bolt pattern". They were too short and did not have enough engagement with some sure grips, the inboard end of the axle was sheared off. I too limped home with one wheel drive.....

You need to have a look in the diff, and if there was any shock loading (or unloading) you also need to inspect the sprag in the transmission.