On the news last week I saw that they required 4WD vehicles chain up before going over a pass here in Washington. I checked the Washington State Department of Transportation and found the following (please read last sentence)...
Exception for all wheel drive vehicles: when "chains required" signs are posted, all-wheel drive vehicles are exempt from the chain requirement when all wheels are in gear and are equipped with approved traction devices, provided that tire chains for at least one set of drive tires are carried in the vehicle. However, the patrol can require chains on all-wheel drive vehicles as well, if conditions warrant their use."