You already have a free code reader in your truck. Cycle the key off-on-off-on-off-on (3 times without starting, within 5 seconds). The codes will appear in the odometer window, one after another followed by "done". Be sure to differentiate between "6" and "b".
I bought one of the $20 blue tooth readers from Amazon. I use it with my android phone and the "torque" application. It works well - reads and clears codes, reads sensor values, etc. It only has the "public" information, so it can't read any of Dodge/Cummins special stuff. Based on the reviews on Amazon, getting a good reader is a crap shoot, I think this is one of the rare times I won the lottery.