Put the lever into the 4wd position.
there should be a button on your left of the steering wheel and if not, it doesn't have 4 wheel drive
I would start by checking vacuum lines
The 2001 BMW M5 has rear wheel drive.
The 2001 Acura TL has front wheel drive.
The 2001 Audi A6 has front wheel drive.
The 2001 Ford Expedition has rear wheel drive.
The 2001 Toyota Camry has front wheel drive.
The 2001 GMC Yukon has rear wheel drive.
The 2001 Subaru Forester has all wheel drive.
The 2001 Audi A4 has front wheel drive.
The 2001 BMW X5 has all wheel drive.
The 2001 Ford Mustang has rear wheel drive.