your best price would be to find a Mercedes wrecking yard, the rear diff is easy to pull out as a unit and reinstall in another car. Most mercedes come with highway gears, some are close to 2.7~:1 almost more than you need. I would search ebay for a carrier, or a whole rear diff, if possible clk32 clk55 are mucho beafier than the crossfire, srt6 or slk32, and the aftermarket offers a posi for the clk cars' diff.

if your a good junkayard dog, there is a MBZ 1990 station wagon that has a posi unit that should bolt up, and then there is the diesle wagons they have an electronic locker, but gear ratio it numerically too low, like 2.60, yuk
you can also check out the earlier ml320-55's since they are designed for off roading.


also curious to find out how you manage to program the speedo.