Is it safe to do so at fwy speeds?

Well you don't want to do it on purpose, but if you did it by accident you most likely didn't hurt anything. Almost everyone with 4wd will do it at some point by accident.

A similar transfer case and front differential are used on the Hummers and they are running 100 percent four wheel drive. That should give you some confidence.

As for mine, if the conditions of the road are unpredictable, into 4 wheel she goes. Most dirt roads are also in 4 wheel and 50+ mph.

You didn't hurt anything but best by far to get out of 4WD on the pavement.

when I had 4x4 I did use this on the highway,(very icy at times) but never over 55 and always be careful when turning.

As the manual says it will cause premature wear if you use it for extended periods of time when not nessisary in situations like that.

premature wear is about all that can happen to the truck, but it isnt very safe, except for ice where 4WD is needed. it can cause some handling quriks. also just so you know, the T-Case in a hummer is TOTALLY different. the H1 hummers basically are ALL WHEEL DRIVE, not 4WD. the t-case has clutches in it to allow slipping when cornering, when HI Lock or LO Lock is selected it acts like a true 4WD and will hop around while cornering on dry pavement

I ran mine once in 4wd when truck was new, almost 45 minutes on state roads towing 8.000 lbs. No problems for 60k. Sometimes hi tech buttons are not good....lol.

No worries, some guys run 4wd at the strip at quite a bit more than freeway speeds. Wouldn't recomend making a practice out of it.

Ive probably done it myself at least twice for ~60 miles each time after leaving the sand dunes. No problems yet.

My old 77 had a had a habit of eating rear drive shafts.. I think I spent as much time driving in 4x4 so I could drive the truck home in fwd as I did in 2wd. Thats got nothing to do with anything that is being talked about here really, but this thread made me think of it. :)

In the front, almost everybody has open carriers that will compensate when turning. When you go to running a front locker is when you will notice breakage. At the very most, you will just put a little extra wear on the u-joints of the driveshaft. No big deal really.

You will never be able to replace all that expensive diesel fuel you wasted!