Best way to use the 4x4 of a 1988 Sierra?
 

I enjoy driving my 1988 Sierra pickup truck, and often have to engage all four wheels for driving to work and back home when the roads are icy. Sometimes the roads have a combination of dry spots plus icy ones, and the usual driving speed for me is around 55 MPH to save a little fuel. But I was wondering if it's safe for the front axle and gears keeping the front wheels engaged most of the time, specially at times when the road has long stretches of dry asphalt (no ice or snow).

My coworkers tell me that they leave their trucks on 4x4 all winter long, but I don't really know if this is a good idea.

I would appreciate any input, hints, etc., that you may have on this subject, since I want to keep the front axle and gears of this old truck in good working order.

Thanks in advance, and have a wonderful year 2013.