If it has 862 or 706 heads (can be seen by a casting mark on the front of the cylinder head by the valve cover) its a 5.3 or 4.8. If its 317 its a 6.0
Theres no reason it would be replaced with only 50K miles.
I wouldnt worry about the drivetrain. I would worry about the doors, interior, and suspension. Rail yards are not kind places and its a company truck so its going to hit ruts at 40mph with no care in the world. It also is going to get the doors left open, windows rolled down and rained on inside because its a work truck and "not mine" so nobody cares.
If it doesn't knock and there is oil on the dipstick its probably fine. I'd check more if the shocks are leaking, if the tires are worn straight, how much slop in the steering there is, how bad it rattles down the road, if all the heater and AC blend doors work.
A lot of those HD pickups also have an hour meter built into the odometer display. Try cycling through it to see how many hours it has. It could have 50K miles and 5000 hours of idling.
If its one of those service trucks with the inset wheels and the bumpers that have liftable rail wheels so it can drive down a track run away. My bet is its one of those and the rail crew pulled the service bed and rail axles off and sold it at auction. Someone else buys it, changes the wheels and throws a rusty bed on it from the salvage yard to sell it as a low mileage truck.
