sounds kinda stupid, but the first thing I would check is under the truck.
1. Since you say its a new rebuild, maybe someone forgot to put the pin in the shifter cable, something from the road came up and hit it, knocked the trans in neutral and the cable disconnect??
ok ok ok, but it doesnt hurt to check.
2. Next I would check for any visable leaks or cracks under the truck.
3. Pop the hood and check the fluid. You should have some fluid on the dipstick reguardless of it running or not, I would check for debris in the fluid and smell if its burnt or anything.
4. Call up the trans shop it was rebuilt at and explain to them the problem and that you will be bringing it back for them to fix correctly this time.
5. Any chance you have a TCS converter in there?? A friend of mine had one in his Buick, was traveling at the same speed, encountered the same problem, BANG then nothing. Turns out the center spline where its welded to the converter ripped right out for no apparent reason. Thankfully nothing got in the trans so it was just a simple converter replacement and he was back on the road.
hope that helps.
