I can't exactly remember if your year of truck had it or not, but some of the V8s had a flapper thingy on the passenger side of the exhaust manifold exit. I think it had a thermal actuator that, when closed, forced the exhaust gas across through the intake manifold, warming the engine up quicker. I wonder if it's stuck in the closed position? What you're describing sounds like exhaust restriction, so if that isn't it, check your mufflers. If you had a catalytic converter, I'd suggest that, but alas, we ain't got no catalytic converters on THESE trucks!
