The lower tow ratings on the manuals has to do launching on a hill. The relatively tall first gear ratios and lack of off-idle torque in these types of rigs make launching on a steep hill murder on the clutch. By comparison, my F250 diesel has a super-low first gear and a lot of off-idle torque, so I can launch a heavy load on a hill without slipping the clutch.
I apologize that I didn't see what your price range is, I was going to suggest looking for a first-generation Toyota Tundra, but a quick search shows those to be almost ridiculously expensive.
