just like everyone else is saying, 4HI is to be used in low traction situations, use your judgement. If you are driving on snow in 4HI and you hit a plowed section, your TC aint gonna blow up. IF you are in a high traction situation, keep your wheels straight.

 

Here's an interesting reference point. Chevy claims their new Tahoe Police Package SUV has a four wheel drive system that will safely operate up to 138 MPH. Although the drivetrain might be safe up to that speed, I shudder to think about somebody actually driving one in marginal traction situations at that speed.

 

I used to have an 08 lariat and I put well over 50000kms in 4hi and at well over 100kmh over the winters and never once had an issue... Just service your front diff and T case more often is all .. Argue with me all you want but I did it and nothing happened...

 

I did 99 mph in 4 hi at the track. I put it in 4x2 after the launch but it doesn't disengage 4 hi until you let off the throttle at the end of the track.
I ran a 13.5 in the 1/4 mile.