The company
http://www.kamitiro.de makes a 4wd conversion kit for Marder and such...
List price € 1499,- without body, wheels, engine, exhaust and rear dif(guess from looking at the parts list)
http://www.wws-racing.de had announced a WWS Fox Pro 4wd version with a kardan drive, but I have last week received information, that due to lack of interest on the German Market, they do not plan to make any more 4wd versions.