We offer a Wavetrac, Eaton, or US Gear diff for the 12 bolt rearends, but the problem of the bearing spinning on the diff needs to be addressed.
First, you need to make sure the caps are installed on the correct side. There should be a marking on the cap and the housing to be sure they go back where they came from when the housing was machined. If the caps are inmstalled wrong it will put the bearing bores out of alignment and possibly cause the issue you have. Then when you go to replace the diff unit, be sure to use both new bearing and races and apply some Loctite to the mating surfaces.