Actually, for me it was a late model 'rear clip' (frame cut off) with the entire axle swapped out.

From what I have seen (and I am still a novice at this), as long as you have the same bolt pattern on the pumpkin, same ring gear diameter and same axle splines, you should be able to swap from a single speed to a two speed differential. I have seen these listed on the truck sites (LKQ, etc.) for as low as $495, with a variety of ratios, usually in the 6.50/8.90 or 7.17/9.82 range, but there are some high speed ratios like I got in the 4.11/5.63 area.