rear locker
BUT thats down the road a bit, because if you want bigger tires then you need different gears and you mine as well do the gears and locker together.
So why am I posting this? To re-confirm what Rick said.
With a 91 I would first start by getting it back into tip top shape, then worry about taking it off road and making it more capable.
Your off road driving abilities will grow with the build of the truck, so starting small and working your way up is a GREAT way to go.
31" tires, a rear locker, and a good driver can take even a stock 4 door Ex places you never dreamed!
there is no one way to build an explorer, every single truck build is different, and its good that way.
We can come on here and give advise and tell you what we did , why and how we would do it different next time, but at the end of the day it all depends on you and your truck, what you like and where you want to go with it.