During my (somewhat) routine underbelly inspection I found that this guy (steel wheel in the center of the photo)

had come loose, practically ready to fall right off! I tightened it back up, of course, thinking it may be the source of some of my Rover Clunks. (19mm, if anyone needs to know.)
So now: what the heck is it? Consulted the RAVE but couldn't find anything remotely similar.

 

2 things come to mind either it is there to protect something or it is a vibration damper

 

It bolts up right next to the front differential, and it's solid steel, so you're right it's probably some kind of vibration damper. Haven't driven the truck since tightening it, maybe it'll make a difference...

 

it is a harmonic dampener, designed to change the harmonic frequency of that part of the front end by adding weight to it, sort of like handlebar end weights on a motorcycle. I would imagine it is there to reduce vibration or howl at highway speeds. Lots of people think it unnecessary, and it seems to be pretty much the first thing to be ripped off the underside after a rock encounter.

One thing to note; mine has a safety wire holding the bolt in - you might want to investigate that being that it looks like yours is missing - that thing weighs a good 15 lbs and would do a number on another car/person in the car if it came off at speed and started bouncing down the highway.

 

Mine still has the safety wire. Difficult to see in the photo, but it's at the top.
Thanks for the response!

 

Did some more research and found a bit more info here:
The page has some interesting side-by-side comparisons of D1 vs. D2 components.

 

Learn something everyday.

 

Also worth mentioning is that in a pinch the rubber mount for the harmonic balancer can be taken out and used for an engine or transfercase mount. This is because they're the exact same part.

 

All I know is mine is LONG gone. Never missed it. Very slight vibrations are the least of my worries on a 3" lift and 33s. Slight vibration is just a fact of life on my rig.