Diesel Bombers - Transmission/Truck Problem

I am *having problems with my *1997 Ford F250. *To start periodically when driving at around 65mph or 2500 Rpm in 5th gear the truck would begin to shake. If i press the clutch in an bring the rpms to the same spot the truck would shake. It seemed as though it was coming from the engine. It would go away if I would simply pull over and turn off the truck for a minute or two. This problem does not happen in any pattern. Soon after when the truck was running fine I would see me indicator saying I was in 4X4 high even though the transfer ase shifter is in 2 High. This continues to happen and even if you push the shifter the light stays on. I took it to the shop to have or looked at and the mechanic said the the transmission casing is cracked and the transfer case is hanging loosely there as well. I am at a lost I do nt know how it cracked or where to look for a new one the truck is stock and just had a new clutch put in. I've called a couple of local junkyards with no luck. Any advice ?*

Thanks