If the timing belt isn't moving, it is either broken or the teeth have stripped off at the crankshaft pulley. A broken timing belt will usually show signs of fraying at the camshaft sprocket. If the teeth stripped at the crankshaft pulley, which is very common for these trucks. Another but not very common possibility is the crankshaft key way could have stripped. This is very rare and usually is followed by a bad repair.
You will need to do some disassembly to discover what has happened.
I recommend the following inspection to help you out; Timing belt replacement.
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