Like Gary, no damage observed here in many miles of 4x4 at any reasonable speed on slushy snow over pavement, or running fast on gravel. However I have occasionally noticed the unique smell of the manual transmission getting warm, which is a clear sign the tires aren't slipping enough to allow stress to dissipate. I then shifted back to 2-high between slippery spots.
My feeling is that if it improves safety, it is worth the increased cost for fuel at the time and the probable increased driveline wear.
My old 4-speed V8 Wagoneer (sold at 138 k miles) and present 5 speed Isuzu Trooper (16 years, 130 k miles) both saw quite a bit of this without any apparent consequences. I would expect the U-joints to be the first thing to loosen up but they never did. No driveline problems or increased noise in either vehicle.
I think the recommendation for 55mph might be related to squirrely handling running at higher speed with the 'part-time' (no center differential) 4x4 engaged. The Wagoneer and Trooper handled predictably in a slight slide on a curve, but in between the Wagoneer and Trooper I had an AMC Eagle (4x4 station wagon) that was terrifying if any tire slid the slightest amount. At that point you could control the radius of the curve with the throttle but nothing you did would affect which way the nose was pointed. That vehicle should have had its own unique squirrely handling warning sticker.
I'm sure others' advice will be different....
