Get help to push the truck out of the way in neutral.
Drop the drive shaft from the differential if you must.
Call a tow truck.

You need to put a breaker bar on the crank shaft bolt and attempt rotating the engine.

The flywheel may be broken?
A transmission pump can seize up. (automatic transmission).
A manual transmission can (under certain conditions) lock in two gears at the same time. (manual transmission).






Have a great day.

__________________
ASE Master Mechanic

Prototype R&D/testing:
Thermal & Aerodynamic System Engineering (TASE) Senior vehicle instrumentation technician.
Noise Vibration and Harshness (NVH).
Dynamometer.
Heat exchanger durability.
HV-A/C Climate Control.
Vehicle build.
Fleet Durability
Technical Quality Auditor.
Automotive Technical Writer

1985 300SD
1983 300D

https://www.boldegoist.com/