The comments about the case being in a bind from unequal forces acting on the rear and front drive shafts are correct. It's the same principle as to why a front selectable locker (such as a Zip Air Locker or a Toyota E-Locker) won't unlock if in a bind. Rocking back and forth is a good method to free the drivetrain, but if you have to reeeeally force it, then don't. Just keep driving for a few more mins and then try it again. Either it will disengage 4WD normally or it won't. I also believe that Twin Stick equipped rigs are more likely to disengage 4WD (I do this often to help turn sharp corners) so it's possible that you just never noticed it as much before.
Larger tires and/or airing way down will also add resistance to the front end which will require more time for the stresses and loads to naturally work their course.
Regards,
BigMike
