Very interesting discussion. Makes me want to throw in my 2 cents as well:
On fitting the FJ80 body on old shape Full Size Wagoneer/Cherokee chasis: Yes, anything is doable, the real question is that is it worth the effort and expense. As Ehsan points out the two are not a simple match and there would be an extensive number of jugaars all over the place to make ends meet. Practically speaking, there is a 90% chance that the end result would be an aesthetic and visual disaster, and even a ten year old from across the street would yell "body change".
Nevertheless, assuming that you spend 1 year getting all the hooks and nooks right and pay 100K+ to your denter on top of the chasis, body other parts of 300-500K (depending on what you put in). The car that you would have in the end would still not be worth more than a good newly restored Wagoneer's market price and will not sell as an FJ80. Most importantly, 3-5 years down the road you in all likelihood be able to hear those body jugaars play a U2 song from the rattle and hum album everytime you drive on a normal Pakistani road. In short, the car will not sell well and will not last long.
On the legality of the matter, not sure if I follow everything said above, but I beleive that as a rule while body parts are changeable, full body changes involving chasis plate tampering is asking for trouble.
My recommendation, if you really want to work on a Wagoneer, would also be to fit in the drivetrain and diffs from an FJ80 onto an old Wagoneer body while keeping the original leaf spring suspension with some good gas shocks. The somewhat tricky bit is restoring the Wagoneer body but with a good denter, some parts from Quetta and a nice paint job, I would think a handsome and solid result can be produced.