Grant, The reason they don't allow full size trucks is only partly because of the wheelbase, my truck is 125"WB with 5" of lift and 33's and it's too long.
The big problem is the W I D T H . A truck thats six and a half foot wide won't fit between trees that are spaced at six foot, period, end of story. Also, you'd never be able to make it around alot of the corners. They let fullsizers in for a while and they were tearing trees down and making a mess of the trails.
Go to the link and check out some of the pics. Matts sport has about a 105" WB? and look at it on the jeeptrap. Wouldn't your dads fullsize look good sitting on the framerails with both sets of tires hanging in the air? The trail is one of those after another, theres no way around them.
If they have to bring the endloader out to get you out of a stuck it's $100.00
http://albums.photopoint.com/j/AlbumIndex?u=1153055&a=9408182
Hope this makes it a little easier to understand.
There are a few places that fullsizers could play, like the quarry, but in the past the ones they allowed in wouldn't stay where they were told to and ruined it for all. I know, it sucks, but thats the way it goes.
