Quoted:
Looking to spend about 6k and keep it under 150k miles. I like extended cab 4x4. But not really picky on color or features.
Find a time machine. That's about your only option. Prices aren't going to vary that much regionally. You might find something a couple thousand cheaper in another state, but you won't find a 150k mile truck for $6k. The EPA has pushed a lot of new regulations in the diesel pickup market, and new trucks are $60k today. That has caused the used market to see huge demand for trucks and prices to stay high. You can buy a new diesel full of new emissions equipment for $60k, or you can buy a used model that's exempt. Most people have chosen to buy used.
For $6k, you're probably looking at something that's 20+ years old and 300k+ miles. Around here, even a 10 year old diesel with 150K miles is going to be $10k+, regardless of make/model/options. Late 80's Cummins will usually sell for $5k-6k, but most of them are 200k+ miles.
ETA: The Dodge you posted would probably sell for $8k-10k in most parts of the country. I would guess the one in the ad either has something wrong with it, or it will be gone within a few hours.
How much does your chipper and/or dump trailer weigh? A 3/4 ton truck with a gas engine will generally pull the same weight, just with less fuel efficiency. But you'll save $5k+ and unless you're towing on a daily basis, the fuel cost won't outweigh the savings.