1985 300SD and 1987 300SDL rear differential
 

The gear ratio is the same at 2.88. Are they the same and interchangeable?

anyone?

I would say they are if they bolt up. not 100% on this though.

its different the 85 SD has the axle shafts going into the diff and you need to take the diff cover off to get the lock clips out to pull the axles (thats how it was of my 85 SD). The 87 SDL has flanges on the diff and the axles bolt up to the flanges on the diff side. If I recall the mounting points are different as well but don't quote me on that.

W126 81 to 85 first generation, and 86 to 91 are second generation.

First gen rear suspention is the same as the W123 rear suspention.

The second gen W126 is a different mounting on the differential.

The flanges mentioned might be the bolted on weights, but I suspect the second gen uus the Annular and or Homokenic type axles.

Bottom line is, the second gen will not directly bolt onto the first generation W126.

Charlie