Note: I know next to nothing about cars.

I brought in my '86 300E in for preventive maintenance today and had a lovely surprise waiting for me when I picked it up. Apparently the clunk my transmission makes when shifting into gear is my differential dying. Can anyone give me an idea of how much it the parts cost to replace and how much labor I can expect to be charged for? Also, how long is it "safe" to drive a car with this issue as I do have a road trip planned in a couple months.

Thanks in advance for any replies.