Geroge,
Electronic
Shift
On the
Fly. With the 4x4 system in my truck or any of the 99 and up trucks. The have an auto/manual lockout on them. I can turn the "knob" on them and lock them in 100% of the time just like good old lockouts. However, if I have them unlocked, and I turn the switch in the cab to 4x4, vacuum is applied to the lockouts and they will lock in, the t-case shifts and I am in 4x4 until I turn the switch back to 2-hi. This is really nice in heavy rain and snowy conditions, that if needed while running down ther road I can switch to 4-hi and everything locks in and I am in 4x4 even if the lockouts were unlocked. The down side of this system, these axles use a unit bearing hub, not a "spindle and bearing" hub. So there is a replaceable unit that has the hub and bearing as an unserviceable unit, you just swap everything out, which does give you a new "everything" in there, but 1 unit bearing unit is abour $300. On the D-60 for example, you can buy replacement bearings for both sides and all the accompanying seals for maybe 50 or 60 bucks for the whole axle, and you are good. To replace the same "bearing" in my truck would be 6 or 700 bucks......
There is a company that makes a conversion kit to goto the "old style" spindle and bearing setup but that kit is something like $1500. That seems steep, but they use a standard bearing so you can get them cheap..... so if you are going to keep the truck for a long time, you may actuall come out ahead if you get the conversion kit, because "rebuilding" the front twice is about the same price. The ony bad thing is, this kit uses standard lockouts, so you loose the ESOF feature, but you do gain the ability to use 2-lo which I do not have the ability to do......