Body swap?
 

How difficult would it be to move the 1st gen rx7 body to either an s10 or a jeep cherokee ?

easy....just get a chainsaw..and a couple buddies..case of beer..Done!

Ha
 

You are more or less looking at making the car into a truck...Putting the body onto the truck frame.
Yes,it can be done,..lots of 4x4 guys love doing this with say Camaro onto Blazer frame type builds.
What you have to consider is the whole mating thing,how you are going to tackle placing the car body onto the frame and making the suspension and steering work.
Brakes are easy as they just involve hoses but the above mentioned is a little more involved.
I can only venture that you'd have to pretty well cut away any shock towers etc and start boxing the body up to make mounts,make plates to mate the body to frame..leaving the cabin alone to retain some sort of place to sit.
In other words alot of cutting,welding and man hours.
Where there is a Will.. there is a way...OR a dead guy giving some stuff to his living relatives!

The funny thing is, Cherokees are all unibodies too...