I wonder if the hubs are really disengaging the whelels from the front axle. Starting at 40mph seems like something is wrong, but I had an old Jeep that would start vobrating if I left the hubs locked and drove above 75 or so. This was an old grand wagoneer, so it had a viscose coupling in the transfer case so it could be driven in 4-hi on pavement. I always figured it was just drivelide imbalance in the front.
Maybe its worth taking a strait edge and checking to see if one of the front half shafts has beed bent very slightly. Who knows, maybe someone had it towed and the towtruck driver hooked up to the shaft to tighten it down on the truck (I'm picturing a flatbed tow truck). It stands to reason that if the axle was pulled on hard enough to bend it even slightly, then maybe the hub or hubs are damaged too. You can check to see if the front drive shaft spins wen you roll the truck forward a bit. Just take a look while one of your buddies drives it for a few feet. It's worth just taking a look.
Good luck, this is a good one.

Johnny