What is the service interval for differential fluid. I have 130K on my 84 300SD, have changed the fluid once at 100K.

Mercedes doesn't have a schedule for servicing the rear axle for cars newer than 1980. However most shops will suggest a fluid change at 30-60k miles. Plan to keep the car-change the fluid.

While you're at it, consider cleaning the vent on top of the differential. It is kind of like a post with a bottle cap crimped onto the top. A box wrench (don't remember the size) will remove it. Soak it in diesel and blow it out. I understand that a clogged vent can lead to very expensive repair bills.

You should change the fluid in the differential every 60K for maximum life. Use a gear lube with a API rating of GL-5 or 75W-90 viscosity rating.