On the 99 Explorer you do have rack and pinion. You could have an outer tie rod loose (like previous stated) or a ball joint loose. On the 91-94 you have a gear box that wears out and the problem also could be a inner tie rod, outter tie rod, a loose gear box, or ball joints loose. And if you have a loose ball joint or tie rod there is no way to tighten those up, just replace them. But if you have a gear box loose you have two options. Buy a new one for aproxmily 500 bucks or you can tighten it up. To tighten it up: you need to loosen the jam nut that is on the stud that sticks out at the top of the gear box. Then tighten the stud only a quarter turn at a time. Over tighen will make is dangerous to drive. Then when you are all done, tighten the nut making sure you hold the stud, and then go for a test drive. The adjust again if needed. But only go alittle bit at a time on the adjusting stud.
Eric