4WD is really two wheel drive plus - 2WD is one wheel drive plus. I drove for two years with the hubs locked in - no ill affects, including driving 14 hr+ in 4wd at 80mph. MPG suffers, but at ANY SPEED you can toss it into 4WD instantly as long as you are not spinning the rear tires. When the hubs are locked everything is spinning up front as fast as the rear axle, meaning the t-case gears are going the same speed. As long as you are not skidding one axle or spinning the rear tires you won't even feel yourself going into 4wd.
The problem is the stupid Ford Sterling axles have absoulte **** for aftermarket support in the traction area. If there is an offering available, its expensive - too expensive. It would be cheaper to toss a 14B in the rear due to the locker availability and obvious strength.
If you want true 4wd you are going to need at least limited slip front and rear. I've been very happy with my trak-loc on and off road as well as any gear type torque biasing device (tru-track). Detroits can act funny - but they are predictable on what they are going to do.
Open diffs provide power to the wheel with the least resistance - one trick is to ride the brakes to equal out the actual resistant felt by the axle to get both tires to spin. It somewhat works, but anything mechanical is going to work better.
If you take anything away from this - remember these things.
You can totally drive around year round with the hubs locked in - it won't hurt anything. I've driven all of my manual hub vehicles at freeway speeds with no problems - it just hurts MPG's.
You can totally toss the truck into 4wd at any time as long as all the tires are spinning the same speed - if you lock up one axle (poorly maintained) or are spinning the rears it will grind the gears. It probably won't damage anything, but not a good idea.
If you want more traction you are going to want to add a locker of some sort. I've seriously considered putting a lunchbox locker in the front axle as when you want 4wd, you want 4wd and a locked front with the diesel over it will hook up every time. A locker out back is less useful as there isn't much weight over the rear tires.