85 rear axle swap from 8 inch to 9 inch

[82 GT]

A much easier swap would get an 8.8 rear from an '87-93 5.0 mustang. It's a direct bolt in and very strong.
As far as the explorer rears you've read about....the carriers inside the explorer rears are 31 spline, which bolts into the 8.8 pumpkin. You can't use the explorer exles because they are different lengths; you would need a set of aftermarket 31 spline axles.
Doing a 9 inch rear swap is a little more involved but not impossible. They sell 9 inch rears already setup for the foxbody mustangs with any gear ratio you want but cost around $600-$700 dollars.