Welcome to part-time 4WD. That is what you have and what you are describing is normal drivetrain binding. I assume you were doing this 4WD test on a high traction surface like pavement? If so, that is generally to be avoided for just the reasons you describe. To shift out of 4H, try driving straight ahead or backwards for a while to help the stresses unload and then try shifting. Part-time 4WD means you have no center differential in the t-case, it locks the front and rear driveshaft together. To visualize why this can cause problems, pull into an empty parking lot and drive through a puddle of water and hang a sharp u-turn. Hop out and look at the different paths followed by the 4 wheels. The front wheels make a larger arc and therefore turn more revolutions that the rear wheels. That is the source of the binding.