I have to agree with the idea of all axles turning at the same speed and not under load. TMNT......what's the fastest you've were going when you engaged the front axle?
I don't recall the speed, but it was as I approached the end of a paved road to transition onto a muddy, unpaved road. I was probably rolling about 15-20 MPH at that point. It engaged with no problem.
Incidentally, I sometimes disengage the front axle while in motion as well. If I'm on a tight trail, I may engage the front axle to cross a muddy section, or to climb a steep, slippery hill, and then need to make some really tight turns on dry, packed ground. After I get through whatever caused me to engage the front axle, and when the truck is rolling along not under stress, I simply move the lever to disengage the front end. I hear the air release and sometimes a click. I may shift the front axle in and out many times during the course of a 2-3 hour trail ride.
I've honestly never given it a second thought until this thread came up. It just made sense to me, given my experience with civilian 4x4's and heavy equipment. I just apply the same logic to engaging the front axle as I do for changing gears: Don't change gears until the truck is "relaxed." Don't engage the front axle unless the truck is relaxed. The truck is relaxed when it is neither accelerating nor decelerating; the engine isn't pulling or being pushed, and the engine RPM's simply matching the vehicle's constant a speed - no stress in the driveline.
Another thing to consider, if the traction conditions are bad enough that you feel the need to engage the front axle, why in the world would anyone be traveling at 40-50 MPH in 13,000 pound truck anyway? In other words, maybe it is possible to engage the front axle at 45 MPH (I'm not saying it is), but why would you ever NEED to do that?