Yep, my Bronco is now rolling on Rockwell 2.5ton axles and 41" Michelin XL's!!
Haha, just kidding. No, those are over at usa6x6, the 12.5R20 XL's that run about 41"x13" on a 20" rim. He's out of them at the moment, but he usually has them with 70-90% tread for $150 each and dirt cheap shipping. They are only rated to 56 mph but keep in mind they are designed to carry 8000 lbs each so I don't know if that speed applies to a Bronco with 1500 lbs on each tire. Surely we could stretch it out to 65 mph, that's not a bad speed. Norm, you wanna buy some and let me know how they do on the highway?
They are load range F so sidewall strength should not be an issue
If you don't wanna go that big, there's also the 11R16 that are ~38.5" tall and narrow, load range E. The XZL line looks to be more all-terrainish and would probably vibrate less at high speeds, at the expense of voids for mud. Being huge radials, all of these will suck up rocks like no other. For those with higher aspirations (NORM!), all these styles are available up to 54" or so.
Here's the ~38.5" version next to a 38" Swamper:
<img src=http://www.garbee.net/~cabell/december/tires/bradcomp1.jpg>