Good advice. Here's some more that's hopefully helpful.
It may very well be a little pricey. You will need to decide what the dollar value is to you, and how good of a relationship you can establish with this dealer if you purchase.
Check those current tires carefully on the inside fronts for any sign of unusual wear. If they are hesitant to move on the price, just pretend that this purchase is not that important to you and start to walk. In other words be polite but firm that they have to do better. When you reach what you consider to be their absolutely lowest price on the Jeep, hit em up for other inducements......WARRANTY.....repairs on any vehicle can be astronomical these days. Tell them the only way you'll purchase is for them to GIVE you an extended warranty. You have nothing to lose by being semi-aggressive.
Forget all the KBB and NADA stuff they might throw at you. Don't ask what they paid for it, as they are entitled to make a profit on the sale, but not a ridiculous one. Ask them to show you what this Jeep is running through the closest auctions to your area in the little "Black book" or on the Manheim sheets.
And don't forget to also look at your current cars value on those sheets. Don't forget, when you trade you are working two completely separate deals. Don't let them beat you up on the Denalis value.
Also, if they purportedly sold the vehicle, ask them to black out the original buyers personal info and show you the entire work history on the truck. It is important that all required maintanence has been done. And, since the mileage is low, tell them you want a complete change out on all the fluids. The truck has low miles, but is 4 years old. I would at least insist on an LOF, transfer case, and differential change.
Finally, you should be able to see if any TSB's have been performed by looking at the service history. And, I would look at this sight, go over all the TSB's for an 06, and ask them to put in writing that if you discover any 06 issues covered that have not been performed, that they will do so in the future. Maybe ask them to give you 90 days to familiarize yourself with the list and your car, during which time they will perform them. TSB's are generally not covered after the original warranty period unless you have a very friendly dealer.
http://www.wkjeeps.com/wk_srt8.htm
http://www.wkjeeps.com/wk_tsb.htm
Again, be polite but firm, show them you're an educated buyer, but a reasonable person. Don't let them see any overt enthusiasm on your part.
Not sure what trouble driving in the winter means. Maybe the guy just sucked at driving in the snow if you get a lot where you live.
Make sure you get to drive it at freeway speeds, and look for any vibrations or rear end/transfer case whines or noises.
Sorry this is so long, hope some of it helps. Great group of people here, and most are more than willing to help you out. Good luck with your decision.