Even DRWs can porpoise. I don't think being nominally overweight has anything to do with it based on what I've read here and other sites. Porpoising generally occurs because the suspension is in sync from front to back. If in sync, you get a bouncing effect instead.
I'd do the following. Max out the tire pressure in the rear ... it's free. Then try it. If you don't see any improvement, I'd update the shocks. I would go with Rancho adjustables, so you can adjust the front/rear independently. However, Bilsteins are very good!
