You can ride with them on forever if you dont mind the extra (mild) hum, and the extra fuel efficiency cost. I forget to turn mine off sometimes, for hundreds of miles. No harm really. I love it when someone finds this out. They get really exited about not having to stop and get out of the truck.
You can shift on the fly too you know, at any speed! Well, I would at 80 mph, cause you shouldn't be doin 80 in the snow anyway.
4lo up to around 10 mph is fine too, at least according ot the manual of my 89.

Sold Truck: 89 DLX extra cab 4x4 22re Automatic/ New truck 97 T100 4x4 auto Sr5