1986 300E Differential

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.

[engatwork]

Generally these cars will run with a clunking rear differential for along time - thousands of miles actually. Sometimes alot of the differential clunk can be adjusted out by making sure the vacuum system that makes the car shift is in proper working order. How many miles are on the car?

218,000 miles; I bought it from my father in law who had it for a few months. He in turn bought it from a mechanic who refurbed it.