You should not shift into 4HI while moving with either the manual or automatic hubs. You can with fixed hubs only.
If the hubs are unlocked and you are in 2wd, the front shaft and axles are not rotating. IF you shift into 4wd on the fly, it will be a dynamic load on the front drive train and you will break something.
Bob Shaw
superchief87@adelphia.net

87 Raizer, 4.3V6, Mod TH700R4,231CHD, detroit rear and LSD front, bouncy seats, 2" lift and 32x15 BFG MT tires.