How are you planning on using it? There are a few ways you can improve performance for a specific use.
Are you willing or able to do things like cam swaps? A cam swap is a relatively cheap way to get more power out of an explorer. Beyond that, it is a 302, so there are lots of parts available for building engines, but that might be a little beyond your means for now.
I would save up for some headers. Only one company makes them, but they are literally top of the line quality, and they are darn good for our application... but they are going to cost you about $850. They do bolt right up to your stock cats though and will give you a chance to actually use that cat-back you installed.
There are a couple other minor things you can do that aren't REALLY expensive. An electric fan will free up a few ponies, and add to efficiency, but it is REALLY tight, so not many things fit in there. Keeping your vehicle well-maintained is probably going to net you the most positive results.
Swapping from AWD to a manual 4wd transfer case from an F150 can put more power and pep to the wheels, and similarly, you can swap in a 5 speed from one of those F150s...
There are a world of possibilities though for you, I suggest reading up in the modified second gen section. There are plenty of stickies full of information, and tons of threads discussing just about everything you can think of. Don't forget to check out all the stickies in the stock second gen section too, just so you are aware of all the maintenance and repair issues.
Good luck with your explorer.
