Have you checked the following (I simply replace these parts as preventative maintenance):
Spring-end pivot bushings/bolts.
Spring-end shackles sets, including bushings/bolts.
Rear-axle to spring pivot-bushings (inside the cast housing that sits on the middle of each rear spring, and that fits inside a bracket on the rear axle).
Rear axle bearing/seal (on each side - not too hard to remove/replace - you have to pull each axle - not hard, but it might be scary, if you have never done it). This made my Suburban's interior noise quieter, but might not do much for a pickup.
Place a Want Ad here for the anti-sway bar.
Jim Carter sells the bushing set - I think the bushings might be different sizes in different years.
Are all your front-end parts tight (and, accepting grease)?
