I've run highway speeds very often in four wheel drive in all types of vehicles. This was usually on the real cold "sticky" type snow. You can decide what's safe for you.

I wouldn't advise running with the hubs locked and the transfer case in 4 high for any length of time on a hard surface where the chance of wheel slippage is minimal. You're going to end up with some degree of driveline wrap-up which possibly will break something or at the very least wear out your u-joints prematurely.

With my stick shifted 4wd, I leave the hubs locked and just shift on the fly between 2 and 4 wheel when I hit those scary spots.

Do you have electronically shifted 4wd, if so, is that the procedure, that you have to stop to disengage the transfer case? I've never had a vehicle with the electronic shift, but I would be surprised if you couldn't shift on the fly.

Jade97 you beat me to it. I got a phone call in the middle of my typing the post.