On the '93 model there is an electrical switch near the battery. On the '91 model I believe that it is a mechanical switch located in the same area. In either case deactivating the 4-Matic is easy.
I don't know of mechanical ill effects but then again I'm no expert.
All I know is that I ran mine with the 4-Matic off for a couple of months last summer with no problem.
If you send me a private message I can put you in contact with an e-mail group of 4-Matic owners (sort of the 4-Matic AA). Some of the guys on the distribution list would be in a better position to answer your questions.
'93 300E 4Matic
'98 Nissan Altima
'00 Yamaha Road Star Silverado