Skid plate? Did I hear skid plate?
Have a look here
Heads up cheap dealer part - W123 skid pan / skid plate
If you are quick there might be some left - and they're quite cheap (for the dealer) - don't bother with the rubber bushings they come with the skid plate anyway (and if your's don't I'll send you my extra set if I can find them!)
Nice information about the differential. I was wondering if a 3.07 would help. In the end the 300DTs were fitted with a 2.88 (just in case you're interested).
For ride height =>
Mercedes actually fitted taller springs (and possibly spacers) with 15 inch wheels for countries with crappy road surfaces. Have a look at the FSM chapter 32-010 for all of the variations on the theme. The part numbers are listed but I'd be surprised if they are still available.
Here in Holland they sell taxi springs "taxiveren". This guy is usually a good bloke to get in touch with
Mercedes onderdelen voor iedereen | www.SterOnderdelen.nl
I asked him recently for more information about the springs - he didn't send me all of the data I wanted though so I'll have to chase him up on that one...
...I'm a bit busy with other stuff at the moment though.
If you do change the ride height, this thread might help you set the suspension up afterwards
How I adjusted the toe in / out, camber and caster on my W123 300D