Since your used range is 13-6 years old...stay away from half ton's
as they may have some to lots to all of their design margins consumed
by previous owner(s) who towed heave to over the ratings.
That is a very common thing with lots and lots of advice that it is
okay, good for it, been doing it with no problems, etc
Recommend going to at least the next higher class TV in the +8.6K GVWR
range.
Then that the SMOG/EPA mandated controls had the OEMs trying to figure
it all out in the era TV's you are looking for....they are still trying
to figure it all out. Similar to what happened to gassers in the late
70's through the 80's. Gassers are very stable..even GREAT now and hope
diesel's will ditto that soon...
Bottom line in purchasing used is the gamble on how the previous
owner(s) drove it, where they drove it, what they towed and the bigge
how they maintained it
Good luck!