Folks,
I've been driving an 80 Series for 25 years and every situation is different. The vehicle is very predictable with all 3 diffs open. On slippery streets with mashed potato snow/slush, I find the viscous center diff in the 97 causes understeer that my 91 doesn't have. I wouldn't consider locking the center in those situations. You'd end up in your neighbor's yard. Same for hard packed snow/ice. If you're going up a steep hill and are having problems, then I would push the button, but as soon as I was clear it would get unlocked.
As far as locking the center diff, if there is no slippage it can be done at any speed. I do it once on a while on the highway at 70 MPH or so on a nice, straight section of highway. There is no harm in doing this as long as there is no slippage.
If you have a spinning wheel and lock the center it will grind until it engages. Not the best idea, but they're not made of eggshells and dove feathers either.
They key is to learn how YOUR truck reacts in different situations. There is no substitute for experience.
