that scenario can def happen, if they are telling you the truth and the cranks walking you lost your thrust, there is a specific crank bearing that is called the thrust bearing that allows the crank to move back and forth ever so slightly, its only a couple thousandths, not even, it has to be able to move, if it was "tight" it wouldnt move right?? with as many miles as you say you have on the truck its def a possibility, this mostly happens in high horsepower race cars, mine, for example, small block ford, single 98mm turbo, powerglide in a 87 mustang, we always had to check for thrust goin out because of the long duration of stalling the car on the transbrake to build boost to launch, the problem would be that the torque converter gets soooo hot when bringing the car up on the pipe that it can expand, widen, and push the thrust right out. Same thing can happen with real heavy towing, extreme trans temps, or high miles. Get the truck checked out by a knowledgeable diesel mechanic just in case the tranny guys are tryin to pull one over on you, hope this helps