I put it out there since the OP's truck is an '08. Interesting...I wonder when they changed it; 2009 or 2010? I know that mine (2008) will not shift at anything over 45mph. I've tried it on the freeway...nope. I do occasionally shift into 4HI-Lock while going straight down the road in town at lower speeds just to exercise the transfer case & encoder motor (something that is recommended on many vehicles, including my wife's Tahoe). I'll leave it there (no turns, just straightline driving) for about 30 seconds and then shift back; no damage possible in that scenario.

The reality is that our H3's in 4HI are actually AWD, and then when in 4HI-Lock and 4LO-Lock they are 4WD. Hubs are locked in all modes, unlike most 4wd trucks..

 

Thank you, Doc Olds! This was the most useful post here, at least for my purposes. Have a great day driving!

Good news, I guess, is light's been off since Monday. So maybe just a scare. And I learned my lesson.

 

Thank you, glad I could help. Other good info here too.

I was thinking about this yesterday, and talking with a guy a couple doors down who has a new 2017 Silverado 2500HD (nice truck). What it boils down to in simple thought is..... 4Hi Lock is used when traction is so poor that you need lock just to make forward progress.

In my old GMC, you could goof around and leave it in RWD in an open area, but you might be fishtailing trying to keep it moving in deep snow. That would not work trying to navigate obstacles and turning sharp between trees etc... off road.

So, if you are whipping down the highway at 55 mph, no need for lock as you are already making forward progress w/o issue.