i have a t18/d20 from a 78 cherokee. my friend wants the tranny, but can he still retain his factory d20(mated to a t15). what are the differences between jeep dana 20s?

he should be able to. the only differences that may be as far as i know is the the bull gear might be different. you would just have to seperate the two and find out. the bolt pattern on the adaptors could possible be different but i don't think so.

The bull gear might be different, This chart from Novak (http://www.novak-adapt.com/knowledge/transfer_case_gears_grid.htm) shows all the different options. D20 cases had two different bull gears. Same number of teeth, but different angle on them. The only other difference would be the shift linkage. Just swap bull gears when you swap cases and you should be fine.