Man, I hate to see when those rich kids at my school who drive really new MBs abuse them over potholes and speed in a school zone... terrible.....
I think that may be your B2 piston (on transmission) is not broken, but damaged... I read that if the B2 piston completely fails/shatters, you loose all FORWARD gears.... so may be your B2 piston is cracked or split???? Sorry, I am having a hard time figuring out how only 2 gears work....
You have ruled out you linkage... so it seems...
You have also ruled out your vaccum line ... so it seems....
Man, you really go me stumped... lemme see if my owner's bible addresses this issue in the trouble shooting...