1st welcome to the board.
2nd You might get a better field of expertise in the matter if you joined a truck forum like Pirates 4x4 or 67-72 Chevy trucks(had 73 up forum also..)
and to try and help you here I go..
The one from your buddy would need to have the tail shaft changed with your existing tranny in your blazer. So it could bolt to your transfercase and adapter..
If you had time and money, I would look for a newer 700r4 "k" case and have it rebuilt.. The older the tranny of 700r4's the least amount of time it will last with the moving the weight of a blazer..
They just were never that great from the factory in the early years..
Hope this helps