Around here $6k for that truck would be a very fair and reasonable price, assuming there are no rust issues and there's no bad engine noise.
It looks like there may be rust at the rear wheel openings where the flares meet the paint, also while wheels can be replaced, bad corrosion probably means the truck saw plenty of salt. Take the spare off if you can to get a good look at the rear of the frame...if it won't lower that's probably a sign to pass.
Personally, rust would be the main concern for me, followed by making sure the engine isn't making timing chain noise and isn't hemorrhaging coolant from the thermostat housing. If those things check out, truck is probably worth $6K. The SOHC 4.0 has its issues, but I've seen several over 300k miles too.