Some of us are old enough to remember when you had to be going < 5mph to shift between 2hi and 4hi.

I think the first "SOTF" vehicles came out in the mid 80's and used a vacuum differential disconnect similar to the ADD that's on our 4runners. Before that it was either automatic or manual hubs. My 1990 Montero had the automatic hubs and the manual said under 5 mph to shift from 2hi to 4hi. The hubs would lock automatically. To unlock I had to stop, shift from 4hi to 2hi and then back up 5' or so. You could hear them unlock and there were 2 indicators on the dash, one for the hubs (which said "wheel lock") and one for the t-case.