Up until 1989 Dec they are all identical. There may be slight variations such as the type of hub on them and the gearing depending on the market you are in.
In Jan 1990 all the 70 series fronts changed to the high pinion 8" diff, same as the 80 series front. But the brake lines, drive shafts and spring perches are the same.
In 1998 they changed to a coil spring front end here but that is also market dependant as to introduction date.
You can swap back and forth to your hearts content as long as both trucks originally had leaf springs.
I run a pre-1990 front axle and a post 1996 rear axle on my 1990 truck. I unbolted the springs, rolled the front axle out, swapped tires and rolled the other one in, drove it away. Simple.
The bearing packs for the diffs are different. The knuckle kits are mostly the same but the post 1990 knuckle is a tad larger as is the birf. You may encounter a diffence in the pinion flange but rotating it 45 degrees and drilling new holes will solve that. The distance from the transfer case to the pinion flange is the same for all leaf sprung axles so no need to bother with altering the shaft.
Also note that the cable locking diffs use a different side bearing on the carrier support than the open diffs.
Hope that helps.
