It might be your blower motor resistor. I had the same problem in my Durango. It is located near the blower motor. The easy way to see it is to take your glove box door off and you will see wires connected to it.

[img]http://www.imgur.com/hxlGUJt.gif[/img]
4-speed
3:54 Dana
13.01 @107.93 with street tires (not hooking up)