2003 Limited V8 4x4, tow package, only mod is a K&N FIPK. I tow my daughter's high school band trailer which is roughly 6000 lbs loaded, about 20 feet long (it's a converted car hauler). Not an easy thing to tow but well within the truck's capabilities. I get in the neighborhood of 11 MPG when towing, keeping it around 55-60. I have a brake controller connected to the factory wiring and it's the best $129 I ever spent. I only manually shift the trans while towing and pull many grades in third gear. I want to add a trans temp gauge and probably a vacuum gauge, as soon as I figure out where I want to put them. No trans issues so far.
I'd really like to find a set of towing mirrors that work well. I have some strap-on mirrors from Camping World that are barely adequate.
V8 with towing package has been 12,000 lbs GCVWR since 2002, and maybe earlier.
12,000 - 6000 (loaded trailer) = 6000 lbs for loaded Explorer
6000 - 5100 (empty Ex-Lmt?) = 900 lbs for people and cargo in the Ex. My '06 Mounty Premier AWD (empty with 3/4-tank) is exactly 5135 lbs empty. My frame is stiffer (heavier), but your 4x4 is heavier than AWD system. So 5100 is probably close.
So as long as you, passengers, and cargo in the truck, don't add up to more than about 900 lbs, your pushing the GCVWR, but not over.
Strictly from the truck's perspective (6400 lbs GVWR, IIRC):
6400 - 5100 (empty) = 1300 lbs available for people, cargo, and tongue weight
1300 - 600 (10% TW minimum) = 700 lbs
So, 700 lbs is really your limit for people and cargo in the Explorer.
I also highly recommend you get all 3 (4?) axles weight next time you are loaded and out on the road.
If you don't already have it, you should be using a WDH (required for TW greater than 500 lbs).
This will:
1. Push weight back up to the front wheels, for proper handling.
2. Help level the rig (decrease rear sag)
3. Actually push a little weight back to the trailer, helping out the weight on the Explorer itself.