Can anyone tell me for sure if a front IFS diff from an '88 22RE truck will be correct for a '91 truck with the V6?

 

make sure the gears are the same, I think it doesn't make a difference HP or LP.

 

as long as the gear ratio matches the rear diff, and they are the same type(e.g. ADD or non-ADD), they will be exactly the same.

 

As far as the ratio is concerned, I don't care. The reason I'm buying one is to make a quick gear swap. When I say quick, I'm installing a suspension lift & replacing all the bushings, ball nuts, rod ends, brakes, etc. I want to change gearing before I have the truck torn apart.
The only thing confusing me is the ADD thing. My truck as it is now, does not have manual hubs. I am switching to them along with everything else. I'm fairly certain that I have ADD hubs; however, on 4x4wire, where this whole mess is explained, there is no picture of an ADD hub. So, there's just enough doubt in my head that makes me want to verify this even further. If I want to check what the IFS differential has (its removed & basically stripped), what do I look for?

 

The ADD differential has a short extension housing on the driver's side that has a vacuum diaphram on it. It shifts a fork inside that locks/releases the axle shaft, and it takes the place of manual locking hubs. The hubs on this system are nothing more than a flange that locks the axle shaft to the hub, kinda like a front wheel drive hub. It has a small dust cap in the middle (like a 2wd front hub does) that pops off to reveal a 12mm bolt/washer that holds the axle shaft in the hub. I have the ADD system in my truck, and I simply swapped the flanges out to manual hubs. No problems at all! Hope this helps!

 

Illustrations in this thread might help:
https://www.yotatech.com/forums/f116/removing-add-69315/