Check your shocks for wet spots indicating leakage. Check each leaf spring for cracks in any of the leafs. The rear shackles are a known weak spot on early model Titans, so check the bushing area as well as all the parts of the shackle that are welded.
Try spraying some WD-40 on your shackles, I tracked my squeek down to the rear passenger side. Every so often it comes back and I have to hit it again.
One question: Do you have an aftermarket exhaust? I had squeaking from the rubber hanger near the exhaust tip due to the exhaust slipping rearward (loose exhuast clamps). When I repositioned my exhuast the squeak went away.
i found this the other day. the clip that holds the leafs together just behind the rear tire. the side of the leafs were rubbing on it. wd40=no more squeak