time for a change?

The manual that came with my truck doesn't mention changing the timing belt on my truck. Is this something I should have replaced by now? I have roughly 120K miles on my truck. I have changed the oil religously every 3000 to 4000 miles since it was new, but done nothing else to it. It still runs great, so I haven't messed with anything other than brakes, tires, interior and exterior cleaning, etc.

Unless you picked up an odd-ball, you don't have a T-Belt. It'll be a timing chain. Just run it till it stretches too much, or it breaks. That would be why it's not mentioned in the manual.