The Stovebolt Site Forums The Shop Area Driveline Truck lists to the left

I'm not sure if this is the best forum for this question but I'll start here. In the past week or so I've noticed (and my wife has noticed when following me in her car) that my truck is listing to the left a bit. I put a level on the front and rear bumpers just to make sure I wasn't imagining this and, sure enough, it seems to be sinking a bit. Interestingly, I can temporarily correct the problem by lifting the truck at the bed or the wheel well but it sinks again when I get in the driver's seat. It's obviously a suspension problem so where should I look first? Thanks