Found this on another site----------
It was used for two reasons. It was to dampen a low frequency vibration coming from the T-case. If memory serves correct it is 55hz. It also takes the place of the tin washer in the slinger seal. the one that the flared rubber seal rides against. This damper is also used on some XJ's. It was designed by the Hillsdale Tool division of Eagle Picher Automotive, for New Venture Gear in Syracuse NY. I was the engineer that set-up the production line on this part. Two double spindle cnc lathes, and an assembly press to put the rubber in. Oh yeah, a vertical cnc machining center to drill and tap those dreaded puller holes. They were hard to keep in the right position because they are very close to the rim of the hub. I think I have a part at home, and I'm sure I have prints of it still. FYI, there is a very similar damper on Libertys also. I do not work for that company anymore. I may have a picture of the damper if I do a little digging. I'll post it if I find it.
Also found that you really dont need it according to some people, take with a grain of salt. Seems to be a press fit though. Mine has a slinger, not a balancer so not sure.