What rearends will fit under '72 GMC pickup?

Sorry to rear about your rear. I thought that was the problem when I answered your other question. Chevy used corporate (GM) and Dana rears. Yours could be either one. My p/u had a 12 bolt corporate. The cover is slightly irregular. I'm not sure what the Dana cover looks like. If you look on the internal parts you should see GM stampings. To get the same ratio (gearing) accurately count the number of teeth on the pinion (small gear on shaft) and the teeth on the big ring. Taking an accurate measurement of the ring outer diameter will help too. You can find replacements in a truck magazine or website. You might want to have a shop do the work. The components need proper shimming and backlash. Good Luck.