It looks like it's worn more on one side than the other. Is that an optical illusion? If it's not, and is worn more in one area than another, then the distributor shaft has excess runout, causing the odd wear. Not sure what could have caused it, although it could have been that way for a while, and just now wore out.
Also maybe you should check the cam because if the distributer shaft lost teeth the cam could be in the same boat. Then maybe find a reason why. Either way to much movement in the shaft or something like the oil pump attached to the end of the shaft is stuck or dragging.
Did you notice a loss of power from the timing going out of whack or did it go all at once?
What year is that vulcan and mileage because seems to be some pretty nasty oil sludge buildup.
I don't think the balancer would have caused it, I think it's just a coincidence that all this stuff happened at the same time.
Another thing to watch for, as Pat mentioned, is the oil pump drive. I'm unsure if the Vulcan uses a separate shaft to connect the distributor to the oil pump, or if the distributor shaft just goes directly to the oil pump. Either way, that could be making the dist shaft run out and cause the gear problems.
Or, it could be that the car just has a lot of miles on it.
I would just check the gear again after about a week or so, and see what it looks like. Any additional problems wearing out the gear should surface by then.
Also, to beat me, you'd have to do this three times in a matter of three weeks, make two trips to MI and one to OH, and then realize you need a new engine because the block is cracked. Make sure you wait until the dead of winter when it's below 0 to replace the engine too. <_< :lol2: