Limp mode locks the transmission in 2nd gear and it will "bang hard" into gear when you shift it from Park into Drive or Reverse. Also the dash indicator will be just a black square (where it usually shows what gear you're in). If you have all of those things happening your transmission is in limp mode.

Now that we have that straight . . .

1) Check your fluid level. These transmissions are very sensitive to proper fluid level.
2) Remove the TCM (transmission control module), clean the connections and apply dielectric grease on the connector .
3) Check for and clear all trouble codes from the transmission and unfortunately you might not see any ECU engine code from a basic code reader. You might actually need to have a qualified Chrysler dealership (opps I think that may be an oxymoron) or MB dealer clear the code for you. Try to find an independent repair shop that specializes in German and / or Mercedes cars.
4) Lastly, I eventually found that I had a faulty automatic shifter assembly that was putting my transmission into limp mode. The previous owner apparently spilled some kind of liquid into the shifter mechanism. It was occasionally throwing the transmission into limp mode and only rarely threw a trouble code so it was difficult to diagnose.

Don't pull your hair out . . . that won't help. Let us know where you're at after checking the above.