I had a 98 Dak x-cab and when I took it for an overnight test drive I put my '00 Sportsman 500 in the back to be sure it woould fit! It did but the tailgate wouldn't close. No big deal though, I just strapped it in with the cargo hooks in the front of the bed! Another note though, the wheel wells were a hair narrow for the front tires but the bedliner that was in it probably narrowed it a little.
good luck and enjoy both the Dak & the Polaris.
I fit a Polaris Sportsman 500 in the back of my Dakota w/the tailgate closed. Perfect fit. That quad is pretty massive. I regularly haul a Kawasaki Mojave. It's a lot easier to unload than the polaris.