Nick, you want a 3/4-ton or a 1-ton. 3/4-ton will be fine, but if you can get a good deal on a 1-ton SRW you'll be fine with it. Diesel is nice, but you'd do fine with a gas motor. The only difference the engine makes is acceleration, whereas the truck/suspension is what really matters here. I've liked the Ford gas V10s fine in the 200x models. Going older than your desired year range to get a more capable truck is fine and what I would do.
As others have said, the issue isn't the rated capacity. The 3/4 ton trucks are built tougher, better brakes, more stable, etc.
Don't discount a 3/4-ton Suburban or Excursion, I've owned both. I had a 2000 Ford Excursion V10 and that was a great tow vehicle, very reliable. Plus a great family hauler, we're wishing we kept it.
Right now we're in the towing situation you're setting yourself up for - we're towing our ~6,500 lb boat + trailer with Laurie's 1/2-ton Avalanche. Even after putting on upgraded anti-roll bars, upgraded brakes, and air springs, I'd say that it's "ok". It just doesn't have the stability of a 3/4-ton truck, and 60 is about as fast as I'm comfortable going. That's fine since the lake is a 45-minute drive on straight backroads where we're going under 50 most of the time, but if we were talking about longer drives routinely, no doubt we'd be looking for another tow vehicle, and with the kids it would probably be a 3/4-ton Suburban or Excursion again.
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), Dodge second, and Chevy last. The Chevys were also mid/late 90s models and there were significant improvements in the early 2000s.
They make it sound like a 4 cyl. 
