you must have the same ratio diff assigned to the car, otherwise your car will not move at all.
nah, not true, i swop em around all the time in various projects, you need to match them carefully to other things if you want a decent result though, but lower gearing wont always increase acceleration (can produce more wheelspin) and a higher ratio wont always give higher top speed if the engine is not powerfull enough to overcome drag.
When i first had my 200bhp Cortina on the road it was on the standard rear diff, it span the wheels in first and second really easy but went like a bat out of hell from about 50mph up to 110, and then hit the rev limiter at 130, fitted a higher ratio diff and it calmed it down a bit, made it faster off the line and pushed its top end up to 140ish.
As with a lot of mods, its not always simple to get a nice useable car unless you change more than one item.
This is true only on the w210 and newer. This does not affect old cars. There is a thread somewhere on the forum where the guy changed his diff on the w210 and then it would not move. When he got the same ratio diff his car was back to normal
have a look at this thread http://forums.mercedesclub.org.uk/showthread.php?t=35014