Actually, the cheapest way to get more power out of your 5.0L is to throw out everything that is non-essential out of the vehicle to lighten the load. Sure, this is a smartass answer, but it's truthful too.
Explorers generally weigh in at the low to mid 4000 pound range. Drop the extra seats, spare tire, use aftermarket bumpers, less fuel in your tank, etc. If getting off the line fast is what you're trying to do, weight reduction might actually be your cheapest solution. If you don't need it, remove it.
Using synthetic fluids may also allow you to use what power you have more efficiently. Using a 0W-30 synthetic motor oil, as well as synth tranny fluid and differrential fluid will probably give you back a few percent that you're losing with standard fluids, if you're using standard fluids right now.
An electric fan is probably more efficient than your belt driven one, a small change, but it might help.
Mostly just thinking out loud.
