What is sport mode?
you can shift from 2hi to 4hi at speeds below 60 mph if you are going in a straight line and your tires are not slipping. You can shift from 4hi to 2hi at any speed going straight without tires slipping. The centre differential can be locked and unlocked while moving forward in a straight line without tires slipping. You must be stopped and and neutral to shift between 4hi and 4lo.
It amazes me how many questions are asked on here that are clearly answered in the owners manual. The owners manual is an excellent reference and full or all kinds of good information. It is really worth reading (several times!). For those that don't have it, it is worth buying check ebay, or as a last resort, the dealer.