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