A Catch 22 - If you're in slick conditions where you need to engage diff. lock, there's a good chance rear tires will want to push you straight forward and you might not be able to turn (as much/as tight) as the front tires may just plow straight forward. (especially with 2WD's, 4WD helps turn)
So if you need to turn tight, disengage diff lock.....unless this gets you stuck. A bit of a "d*mmed if you do/if you don't" situation.
If one need to turn in the muck, disengage diff lock and use "poor man's diff lock" technique; i.e. use the individual wheel brake to either stop the spinning tire so that the non-spinning tire with traction gets torque to move you out of the area, or use brakes to steer.