It's known as the "death sway", I currently have the same issue, and its a common occurrence for these rigs due to the age of the bushings and or trailing arms. I ordered some trailing arm bushings for my Q but haven't gotten around to putting them on. I don't recommend going over 50-60 mph until you resolve the issue. Luckily for both of us, the fix isn't all that expensive to do. The only trouble I am having is, finding the time to jack up the truck and get underneath it to replace them. If you do some searches on these threads, I'm sure you can find a "How-To" link somewhere. If you have an hydraulic lift on hand or an accessible one, you could do it pretty easily and pretty quickly yourself. Just make sure to research the process before taking things apart.
Here is the Link for your year and model.
http://www.rockauto.com/en/catalog/infi ... g+arm,7504
Cheers,
Javin