This is very common for 4WD trucks. What happens is either the front or rear axle has turned slightly more or less than the other, creating a bind in the transfer case, making it hard to move from 4H to 2H. Doing what Steve suggests above will work. Also, this is most likely to happen when you are on dry pavement at the time you stop. If you are on snow or ice pack, the transfer case bind will release easier because one of the wheels will slip enough to release the bind and allow you to shift out of 4wd. When mine does it I have good luck backing up or going forward quickly, even on pavement, which is enough to take the bind out. Hope this helps...