Don, I'm way new to MB's so I can't quote any specific numbers but I'm not new to cars.
First thing I would do is find out what your stock axle gearing was in the axle you removed and then find out what your swapped in axle has for a ratio. If for example(again I'm not familiar with MB numbers) if the old ratio was say 3.90:1 and the newer one is 3.30:1, in a low HP motor like the 200D that can make a world of difference in 1st gear starting on an incline. You wouldn't notice it too much on a flat staright road at hwy speed though. If you've significantly raised(lowered numerically) your axle ratio you may have to just rev the dog ***** out of it starting on an incline. I'm not saying that's your prob but since you did mention the rear axle has been swapped to an unknown ratio that's where "I" would start anyway. Good luck...hope it's just something easy like a clogged air filter.