Driveshaft issue '79 300sd

[cookieSurprise]

Hey folks, I've got a problem with the driveshaft on my '79 300sd, 189K- one of the bolts that connects the end of the driveshaft to the flex disc/diff has so much play in it, it has made a groove in the flange (see photo).

Two questions: 1: Do I look for a new driveshaft (and where would I look), or is a fill-and-redrill option worth pursuing at a machine shop, and 2: what would cause this to happen in the first place, and is there something else I need to look for to prevent it from happening again?

I'm looking to keep costs down, since I bought the car for $260 due to the driveshaft issue, a bad water pump, and the fact that it looks as though bears have been living in it.

Hoping to wave at all you MB dieselers on the road in 2005, thanks in advance for any and all advice.