Something is wrong with your 300SD...you should get atleast 26 and up to about 28 if you play it chill under 70mph and the roads are not 40% grade!

I can't get 22-24 if I try...

Maybe 24 if I really beat on it the whole tank

If you change the diff and you end up load your engine at low speeds and it is always downshifting...working harder..etc it will not pay off to put in this other diff.

Now if your car sees purely highway miles..it will help.

Mixed driving...you will just hurt your acceleration and your fuel mileage.

I would look at doing other things to the engine before you do this...mpg should be much better.