Off Roading With 1990 Xplorer ~ Any Experience With This?

Blocks in rear, new longer springs up front.
You'll probably need to cut your fenders for any significant increase in tire size.
After jumping through hoops and $, you'll gain a minor amount of clearance under your differential and the ass-end will likely drag on steeps (although a airbag kit maxed out will help).

Aside form the comments/suggestions above, your best bet is aggressive tires, take it slowwww, a second pair of eyes over sketchy terrain, and governor on your willingness to stubbornly forge ahead.