Anyone know of a brake upgrade for the xB that includes swaping the rear drum brakes for discs?

none yet that include a parking brake. Some have done a DIY but they don't recommend it due to it being too much work. Try a search to find what I'm talking about.

moved this to corner carvers

Wilwood is releasing their rear kit with a e-brake in June-July!! Cannot wait. The fronts are perfect! :D