The labor involved in doing this is pretty excessive unless you really need a new differential. I personally would not undertake a differential swap just to get what I consider a better gear ratio.
However, I recently did change out my differential because the old one was severely worn. I should post a picture sometime of just how badly it was worn (and where, so you can all do an inspection).
If you have to get a replacement differential, by all means get one from a 1985 300D with the 2.88:1 gear ratio.
One thing I should advise you about: On my car with the replacement differential, and 15" wheels vs. the stock 14", the trip odometer indicates 9/10ths of a mile for every mile on the interstate. So the speedometer presumably has a 10% under-indication error.
Keep the gear oil changed!
1982 300D at 351K miles
1984 300SD at 217K miles
1987 300D at 370K miles