I have a 2005 Dakota 4x2,and I have replaced the transmission, the pcm, and the CPU. It's still in limp mode. Also, the 25 amp shift solenoid fuse blows as soon as I put it in gear. Could that be the ignition switch?
According to the wiring diagrams, the only 25 amp fuse related to the transmission powers some transmission control circuits in the PCM, and also the solenoid pack and transmission range sensor (both internal to the transmission), through the transmission control relay. If this fuse is blowing as soon as you put it in gear, you have a dead short somewhere. I'm guessing, since you've already replaced the PCM and the transmission itself, it's in the wiring bundles going to the connectors on the transmission. There are two connectors involved, one on either side of the transmission, and they each have seven or eight wires coming into them. Not an easy place to get to, but check those wiring bundles all the way back into the engine compartment and the integrated power module, where all the relays and fuses are.
I doubt very much that the problem is in the ignition switch if you can start and run the engine in park without blowing the fuse.