Sounds like a worn engine. Is the truck high milage? Are you adding oil between oil changes?
Another possible cause, the intake manifold has a gasket that commonly blows out on these engines and sucks oil into the engine. Do you notice the engine "pinging" under heavy engine load? This is a another symptom of the gasket blown out.
I have a 2000 that does the same thing. I notice it more during starting it up more so than anything else. When I do get into the throttle or am hauling something, it doesn't do it much. How severe is the smoke?