Cash,
Hope you have indeed resolved your problem. But if not ....
I have a 97 Explorer, and have had very similar situation. I could sort of feel the pop in the floorboard, and hear it also. It got worse and worse, until it actually made noise just going over bumps at low speed, without hard steering turns. My problem was the steering rack and pinion. After replacement of the unit, the problem was resolved for about 75k miles. Unfortunately, I think mine is starting to pop again. And I clearly recall using a rack and pinion system that was extremely inexpensive, at around $100.00. Maybe should have used a higher quality part.
Just an idea, in the event you have not resolved this.
Good luck.