Here's the symptoms: Truck will suddenly get a heavy vibration while driving, starting at about 15 mph, though the precise speed varies. Doesn't happen all the time, might drive for miles with no problem, might start up right after leaving the driveway. Once started, it continues until the vehicle is almost stopped, then it doesn't start again until 15 mph. Engaging 4WD stops it, and sometimes it returns when I switch it back to 2WD, sometimes it doesn't. The vibration continues even if the truck is clutched and/or shifted into neutral.
Thinking it was hubs or front axle, I disconnected the vacuum line between the manifold and the solenoid, so that the hub locks wouldn't engage, verifying that the front driveshafts spin free while the wheels don't move. Drove it for awhile with no vibration, then switched on 4WD (so the transfer case engages and spins the front differential even though the hubs are unlocked). After switching back to 2WD, the vibration started up, back to 4WD, stops again. This would seem to indicate that the problem is somewhere in the transfer case.