This is getting away from the original thread subject, but I had some experience with "rumbling" tires, and it may be what you have. Please check you tires for uneven wear and read below....
From a previous post, I said:
"This issue has been around for a LONG time. There are hundreds of posts on this on several Ford forums. The general response is that it is caused by poor tire quality.
I have a '98 EB that came with the Goodyear Wrangler RT/S tires. Actually, they sucked on several levels, but I had tons of problems with feathering, or is it cupping? Anyway, it simply destroyed the tires, and caused quite a lot of uncomfortable driving. This all happened in about 6k miles.
I went about correcting this issue methodically, and I am proud to say that it has been solved for 50K or so.
1) First I went in and had a complete alignment. Needed to have a camber kit installed, because i guess Ford didn't think it important to be able to change this (duh).
1a) Complete wheel balancing
2) Then, a few weeks later, I did it again, cuse the first one didn't help. This was at a local shop where I found they did great work!!
2a) Another complete wheel balancing
3) Brought it bake to the second shop again to make sure the alignment was still good. Reason being was the ride was better after the second alignment/balancing, but got worse.
THIS IS WHAT I NOTICED when I was under the truck watching them. There WERE NO WEIGHTS ON THE WHEELS. I knew they put them on, I saw them. I remember some loud bangs, but rocks will do that.
Solution: Tape weights..they hold
Next came new shocks and new tires, and I have not had an issue since. So, it may not apply, but I now make sure that the wheels get tape weights when rebalanced...plus it looks better."
Hope this helps.