If it stays in second gear, that is "limp mode". That is what it defaults to when the Transmission Computer detects a problem. It allows you to drive slowly to the repair shop without needing a tow truck.
It is important it this point to not disconnect the battery or let it run dead. There will be at least one diagnostic fault code in the computer and you don't want to erase that until it has been read and recorded. Once in limp mode, you must turn the ignition switch off, then restart the engine to get it out and back to normal operation.
You may be able to get an idea of the severity of the problem by observing when it goes into limp mode. If you start the engine, shift to "drive", and it immediately starts out in second gear, the problem has already been detected, and it most likely is electrical in nature. Typically that means there's a sensor problem or something with the wiring, as in corroded connector terminals or cut wires. If it starts out in first gear like normal, then bangs back to second gear at some point, it is more likely to be due to slippage in one of the clutch packs. That is due mostly to wear from higher mileage. Jumping back to second gear usually occurs during or right after an up-shift, but it can occur during a down-shift too when you're decelerating.
The diagnostic fault code will tell the mechanic which circuit or system needs further diagnosis. Fault codes never say to replace parts or that they're bad. Also, if he has Chrysler's scanner, it will display a set of numbers called the "clutch volume index", (CVI). Those represent the volume of fluid it takes to apply each clutch pack. Experienced transmission mechanics can tell by those how much the clutch plates have worn down, and approximately how much life, (by percentage, not mileage), is left in each one.