The tranny behind a Intrepid and the 3.7 Dakota trans are very similar in that they both shift automatically, other than that they are very different.

That being said, the dakota trans behind a 3.7 shifts a lot so yes it should be locked in 3rd during anything besides flat land.

I had a 3.9 in my 2000 dakota and it towed just great, I routinely towed up to 6000 lbs and brakeing and handleing even with out a trailer brake was fine (extended cab 4x4 long bed) power on the other hand was puny but not as puny as the 18 wheelers I would pass on the mountain passes. Mine would get around 15 towing on the highway and 20 on the highway empty, I was never one to push my luck and kept the speeds down and did not run wide open up the mountain passes, if you just have to tow in a hurry then no this is not the truck for you but if you are patiant it will be just fine. If you try to keep up with the V8 trucks in the other lane you will suck as much gas as they do, if you drive it like a truck with %75 the power of a V8 your MPG will not be that bad.