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Hi, can you tell me what engine your commander has?
Ok, first thing I'd try is remove the starter relayin the fuse box under the hood. Look for pins 87 and 30 on the bottom of the relay. Take a paper clip and touch it to the 2 cavities 87 and 30 go into. The starter should crank the engine when you do that. Let me know. If it does crank when you do that, you'll need a test light yo procede further.
Ok, do you have a test ligh? We will need that for the next step. Pins 85 and 86 of that relay need a power and ground but if the starter didn't come on when you paper clipped 87 and 30, there's another issue. It won't start and run with the paper clip but it should crank the engine.
By stuck in 4th, do you mean the display is showing it's in overdrive when the shifter is not?
Does it have auto stick? If so, try moving the shifter to the right or left to get the number to change.
So it does have auto stick? Like the + and - to change gears manually?
Ok, if it won't change, it's most likely a bad shifter. If you hold it towards +, it should go to drive. That will also cause your no crank issue. To verify, pin 86 of the starter relay should have power with the key in crank position and pin 85 should have ground in the crank position. I'm betting you will be missing the ground. It comes from the PCM and if it isn't seeing the park or neutral signal, it won't provide that ground to the starter relay. Check those 2 pins next.
Let me know what you find.