The most common cause by far of the loss of the lower blower speeds on this model is a failed blower motor resistor asembly. The power from the blower switch is sent through a series of resistors to drop voltage; blower speed is controlled by sending a lower voltage than 12 volts to the motor to cause it to run more slowly. In hi position, the resistor block is bypassed, allowing the blower to run at full speed if the resistor has failed.
Replacing the blower resistor assembly will almost certainly resolve the problem you are experiencing.
The blower resistor is mounted on the heater case next to the blower motor; it has an electrical plug with (I believe 4) wires connected to it. If you go to the Dodge dealer and purchase a new resistor, you will see what it looks like making it easy to identify. Replacing it is fairly simple; just unplug the electriplug, remove the 2 screws holding it to the heater case, and lift it off.