Clogged radiators happens frequently on the 4-wheelers.

Take the radiator out and soak it in water & degreaser.
You've got to be gentle with the cooling fins, you don't want to bend them or pinch a vein.
Do not use a pressure washer at close range... a hose should be fine.

I takes a couple of cycles, but will get all the mud out eventually.

If you do not get the mud out... you will over heat.

The good news is its a inexpensive fix.... just time consuming.

Here's how we addressed this on the 4-wheelers.
We relo'd the rads up on the front... works like a charm... probably not much of an option for your truch though