Well, I can tell you that I shopped for my 2004 Tahoe Z71 for 6 months; just purchased it last month. The used truck market is crazy hot right now. Demand is high, supply is low, prices are high, and people are putting down deposits within hours of a "good deal" being posted on Autotrader, cars.com, etc. The truck I wound up buying had a deposit put down on it by someone else within 50 minutes of being posted on AutoTrader... and had no pictures or Carfax uploaded yet. (I got lucky, the guy couldn't get financed.) I'm not sure if you're looking for a pickup or SUV, fullsized or smaller. What I can tell you is that 15k isn't going to buy you much in this market if you're looking for a used Tahoe or Avalanche. Silverado/Sierras are more do-able in that price range, or something like a Trailblazer/Envoy. Fullsized GM 4x4s, esp SUVs, are like gold right now.
Personally, as long you're ok with not having the latest body style, I think you're better off with an older, low mileage truck... than a high mileage, newer. Based on my recent shopping experience, high mileage newer trucks aren't discounted as much as older. e.g. I was still looking at $25-30k for 2007 Avalanche Z71 (well equiped) with ~100k miles. Whereas a 2002-2004 with half the mileage was about half the price. Pretty much the same story on the Tahoes, just bump the $'s up another $5k+.