2001 Mercedes Benz SLK320 Slack in rear differential or tra

I have a 2001 SLK320 Mercedes (automatic). When coming to a stop at a red light and then accelerating again I notice a slackness in the engagement. When I alternate between deacceleration and acceleration quickly there appears to be a slackness in the drive train.I also have noticed a very slight oil leak coming from the rear differential. Otherwise when the car is under normal acceleration on the highway there is not a problem. Any idea what the problem is or whether I should be concerned. The car has had it's regular maintenance checks and has 59321 miles. Thanks For Any Insight