If it is out of a 3/4 ton (3600), then the pumpkin should just bolt in. May not help, though. The 67-72 auto (and some manual) trans trucks had a 4.10 ratio vs the 4.57 in the earlier trucks. The carrier is slightly different as well. Patricks sells a 3.90 ring and pinion that will go into a 67-72 carrier. To check the ratio, count the turns at the pinion yoke to make the axle turn 1 rev (if only one end turns, divide by 2). If the unit is apart, count the teeth and divide ring by pinion.
If the axel is out of another truck, it may be usable, but check the width between axel faces to make sure your tires will fit. Accordin to others here, you can get a u-joint made that will connect your open driveline to almost any rear end. Good luck.

Current fleet (subject to change w/o notice)
\'49 GMC 3/4-Ton , 60 Austin Healey Frog-eye Sprite (some assembly required), 2011 Dodge Avenger, 2015 Jeep Cherokee. No, they don't all run.
My other ride is a (B737)no, (B767)no, A320.... Update... was Embraer E190, now Boeing B787.

Knowledge is a wonderful thing, but ignorance means you don't know you can't do something.