On snow, you can go as fast as conditions allow in 4HI with no worries. I've done 80-85 for a good 15 miles (noone on the road but me), just haven't taken a longer trip than that yet. I usually use Auto though, and 4HI for only the worst driving conditions. I consider Auto the "dummy mode" that won't allow me to tear stuff up.....and so I use it as much as possible
Even using Auto, I can easily cruise at 60 when other vehicles are only crawling along at 35ish. If visibility won't allow me to see the slowpokes I reduce speed accordingly. And I don't always go that fast in a blizzard.....just because you "can" go that fast doesn't mean you have to.
Just remember that on snow your acceleration traction is MUCH better than your braking traction and you'll be fine. Watch out for bridge icing.
Now, going in a straight line without snow/ice/rain to allow some slippage (which is mostly what you're concerned with I assume, and not my snow driving lessons), I'm not sure I'd push it past 60....and even then only for a short stretch. For more than that, you want either AWD or FULL-Time 4WD like on some '70s and '80s trucks. Yes, they are different.