Sorry exhaust manifolds, not intake. The Explorer 5.0L has GT-40 intakes which flow really well. You will not need a new upper and lower intake until after you already remove the restrictive factory exhaust manifolds, airbox, and stock exhaust.
Torque Monster headers are the only header on the market that will fit your Explorer.
Ford makes a performance manifold as well, but they are still a 4 into 1 design and flow only a bit better then the factory manifolds.
You can pick up a Ford racing 1" phenolic intake spacer for your GT-40 upper intake manifold though, thats good for a few ponies for cheap.
I used the search button and found this stuff (discussed on this forum ALOT):
http://www.explorerforum.com/forums/showthread.php?t=142741
http://www.explorerforum.com/forums/showthread.php?t=129083
http://www.explorerforum.com/forums/showthread.php?t=152383
Just for starters