Almost all part time 4wd vehicles in the past decade are shift on the fly for 4wd, my Tacoma for example (and more than likely exactly like Supton's Tundra) you can switch from 2 high to 4 high up to 62mph. Now, why any soul is doing that at speed in bad weather is beyond me. 4wd low requires you to be stopped and transmission in neutral.
Goes back further than that. 1996 Explorer, 1997 Expedition...etc all had shift on the fly 4x4.
Even further than that. Shift-on-the-fly 4wd was available on various makes and models in the 80's.
Before that there were selectable hubs that could be left engaged, then it was just a matter of shifting from 2hi to 4 hi and back at any speed. Of course the front axle and driveshaft were always turning with the hubs engaged, whether 4wd was selected in the T-case providing power to the front axle or not.
Yep, can shift up thro 60-something mph. My vintage Tundra was known for bad front diffs from the factory (something about bad preset on one of the bearings; the way to diagnose was to see if the rumble went away in 4Hi while going at a good clip. When the front diff went bad I tried to shift out of 4HI at over the speed, and all it did was beep at me. I've also learned that it won't disengage if there is any loading, like not going straight.