i have seen several SL's that have had this same problem, what is happening is the clutches for the reverse gear are worn and there is too much space between the clutches and the metal discs inside, it can be repaired, but the transmission has to be removed, i have seen this problem damage the reverse piston mb#126-272-22-31 and it has to be replaced, the clutches and discs are available in varying thickness to compesate for the excess space. if you are not going to do this yourself, you should find a competent mercedes shop in your area who can fix this one, or install a factory rebuilt unit. i personally have performed this type repair two times, if anyone sees something i left out, please let me know.
Ray
mercedessd@cox.net