The stock headlights might be "weak" from age. Have you ever replaced the bulbs or cleaned the lenses? The plastic lenses are old enough now that they tend to "yellow", so getting some plastic polish or a headlight restore kit with polish and protectant can remove the yellowed film, and keep them clear and let more light through.
Replacing the bulbs with quality, brand name bulbs might help. Sylvania and Philips are some of the best bulbs to get. You might also consider using bulbs that put out a little more light, something like Sylvania XtraVisions or Philips Hi-Visibility. Stay away from the "blue" or "white" gimmick bulbs like the Sylvania Silverstar though, they have a blue coating on the blub that makes the bulb put out even less light than a stock bulb.
As for the bumper lights, there is a hole about 6-10 inches in from the corners on the bottom of the stock bumper - it's perfectly sized and placed for the mounting stud of aftermarket fog lights.
There was also fog light option on non-limited 91-94 Explorers, and they were mounted under the bumper closer to the center, behind a cut-out valance /air dam panel. You can find these online for not a lot of money (Aftermarket part number is FO1095148), but shipping usually makes it not worth the price. Rockauto has them for just 10 bucks , but they want 5-6 times that price for shipping just because they mistakenly have it listed as a heavy/oversize item. There are some on ebay for ~$40 shipped. These only let you mount some pretty small/slim lights though, compared to using the mounting holes, and you need to make brackets to hold the lights up behind the bumper / valance.
You can also just drill your own holes in the top of the bumper to mount lights up there, or even buy a universal "prerunner" style light bar to bolt to the stock bumper for light mounting, or even just make your own light mounting brackets from some flat steel.