Tough deal.
Here's what I would do. I would find an older diesel 4 door. An 05 or 06 GM would fit the bill but it would max out your budget. (I don't think Dodge made a true 4 door back then? I would stay away from Ford in those years unless you have Fishes luck) :B
I would put a slider in the rear window and then I would get a cap and put on it also with a slider in it with a pass thorough. Then I would buy a big bench seat to bolt to the bed floor. Put a set of belts in it and you're good to go.
I almost went this route back in the day but then one of my kids said he didn't want to go on vacation anymore. Cool, so the project never got off of the ground.
Unconventional? Yep, but that's what I would do.
