They will work on the C4 etc. However, the t-cases they fit are the full size ones. I don't know of anyone running these in a first gen X with a V6. If you have a V8 AWD then you already have a full size trans and they are OD units.
None of these came in a V6 explorer, yet these are all the ford t-cases they fit.
NP 203
NP-205
NP 208
NP 271/273
BW 1345
BW 1356
BW 1356
BW 1356
BW 4406
BW 4407
There are not any options that I know of for the stock t-case and OD from another vehicle. If you have enough money you can make anything fit. I say go for it, if you have money to burn. Just remember how long its going to take to recoup your expenses using OD with this kind of swap. BTW: What kind of mileage do you get now? 21 or even 25 maybe on the highway? I averaged 17 without OD and driving hard/loaded usually.
For a normal person on a normal buget its either manual, chevy, or C5/C4. hammer is considering not even rebuilding a 21 year old transmission.

You can always have the A4LD rebuilt ($1500-2000) and hope it holds for a year or two.
EDIT: For your 91...remember your t-case is either a BW 1350 (late one) or the BW 1354. You listed another number in the post. I have never heard of that one. Hoping its a typo.