An old trick with an open differential in "stuck" conditions when only one wheel would spin is to lightly apply the brakes, which puts more resistance on the spinning wheel and makes both wheels turn.
That and John's suggestions (weight, etc.) will help. But if you run into ice with water on top of it, all bets are off. The only option there is to chain up. I've been in a situation where standing stock still on a road with freezing rain where I slid down the crown of the road.
Adding a locker to the rear of a 2WD truck is an expense that won't help that much. There are situations where even a 4WD with a LS rear can't move. Unless the front diff is also a LS type (no production truck is available that way), even a 4WD is really just 3 wheel drive.
