If it has seen the red end of the temperature gauge more than once, you can safely plan on replacing the head gaskets.
Other than that... removal of thermostat is rather a useless "last resort" deal.
Make sure that
- the radiator is free-flowing (IR gauge may help); most likely it is half-plugged with scale;
- viscous fan clutch is operational (the simplest way to do it is to buy a new one);
- no fan blades are missing, and fan shroud is present;
- water pump works and pumps coolant around;
- and the system is bled of air properly.
If you're here and the truck is still overheating, it's either head gaskets or new block time.
Out of the factory, D1's cooling system was designed with a lot of cooling capacity - and that resulted in people not maintaining it for many years, and passing barely-working truck to the next owner.