The long and short of it:
I originally had an issue with the front driveshaft u-joints being worn out(ticking noise tipped me off and when I crawled under found alot of slop in the u-joint)...so I pulled it and had the driveshaft rebuilt completely(while the front driveshaft was out I noticed there wasn't a noticeable vibration) and put it back in. Took it backthe next day because the vibration was easily noticeable at 70-75 mph. He balanced it again and gave it back. Took it off and had another different shop balance AND straighten it within 0.005" @ 3600 rpm. Still vibrated at 70-75 mph. Drove from Denver, CO to West Palm Beach, FL like that. About 3 weeks later, found my transfer case leaking between the mid and rear cases. Took it to a great tranny shop and they only found the silicone seal failed; resealed it, rebuilt and balanced the rear shaft just to make sure the driveline was true...still vibrates.

The tranny place suggested I talk to a tire shop they work with...the tire shop noticed the track bar and drag link had play in them that was scuffing the tires very badly and making flat spots that were making the truck bounce. They rotated the tires and suggested I replace the track bar, drag link and seriously consider new tires AFTER I fix the steering. As I drove away with the tires rotated I noticed the vibration was still at the same speed but had moved more to the back of the truck so I'm convinced the tires are trashed. I'm shopping around right now for a track bar upgrade that won't wear out in 5k miles. Sorry this is so long.

 

The only actuator that I can think of would be some sort of wheel lock actuator...like on auto locking hubs. See, our trucks are locked at the hubs to the front axle-train meaning there is no freespin to the front differential. Some trucks have manual locking hubs and others have auto locking hubs...I would imagine auto locking hubs would have an actuator of some sort to lock the hub to the axle. When you shift the transfer case from 4 to 2 wheel drive it disengages the gear that "drives" the front diffand allowsthe front drivelineto freespin. If you jacked the front of the truck up in 2 wheel drive, the tires and front driveshaft would spin.

 

Tell ya what you could do with the truck parked or on the ground...run your hands around the tires and feel for flat spots...the flat spots on my tires are pretty obvious.

 

I had the same kind of vibration at 55mph when I lifted my truck. Vibrated with the front shaft on, smooth with it off. I installed the transmission drop bracket, no more vibration. When I bought the lift from Top Gun Customz, they told me that the bracket was a paper weight and that I should just disguard it. Boy am I glad I didn't listen. That was my problem, and it sounds kinda similar to yours. I dont know if they sell transmission drop brackets for the 2nd gens,but I do know that Sky Jacker used to sell a tranfercase indexing ring with some of their lifts.