1988 300E transmission and front end questions.

I just picked up my car this afternoon from a transmission shop, which replaced a rebuilt transmission I purchased. The history of the replacement transmission is that it was rebuilt to serve as a spare for an 88 300E Brabus 3.6.

On my way home, I noticed that the transmission, short of violently, jerks the car forward when engaging in drive (that is with my feet hard on the brakes). I'm not sure, but this seems to be coming from the differential area. While driving, the transmission is smooth, however, I feel a slight lurch in one of the lower gears (seems like second), when accelerating. The transmission shop tested the tranny before placing it in the car, and they said it was fine. They also performed a road test before calling me to pick it up. My question is, could the tranny have been built for racing, and is that why I'm experiencing this, or is this a sign of a problem, such as a bad transmission, or maybe a differential problem (before handing over the car, the shop owner told me that he adjusted the tranny a couple of times; he is known to be one of the best in the area).

Sorry for the length of this question.

Thanks