I stumbled upon some good advice while perusing the Binder Planet this afternoon: an inexpensive alternative to Dynamat, called Peel & Seal. The commenter claims it’s only about $20 at the Home Depot. I’ll have to pick some of this up next time I’m there.
Update 2.20.10: Struck out at Lowe’s. Doing some reading on the product, I don’t know if I’d want to use this stuff after all. I’m also not sure if the idea is to put this stuff on the inside of the cab or out.