W123, Body

I had my 1983 300D Turbodiesel on car ramps today, to drain the fuel tank and replace some hoses, and much to my horror, noticed that a rear suspension component, whose name I'm not certain of, has a crack. How can this have possibly happened??? What's the proper fix? How would I go about it, and what would I need, etc. The car has about 200,000 miles on it and wasn't in an accident ever. I'm shocked and quite worried that this is a "big project..." Here's some pix... ANY advice would be most appreciated. Doesn't seem like a common problem at all...

I have a two tank SVO system on my Merc with 310,000 Kms on the clock - 100,000 Kms are mine most on SVO. I have never been stumped, held up or broken down before. The engine starts and shuts down on diesel. The problem- I was away for a week and my Merc sat idle. When I came back the Merc started hesitantly (low battery). I drove about 20 Kms and the next day she would not start for love nor money; totally dead. Not even a kick from one cyllinder. I checked; Lift pump ok, replaced with electric lift pump & still no go. IP moving fuel when lines cracked. Cam timing marks and harmonic balancer marks line up ok. Valve clearance not binding, Glow plugs working, has the Mercedessource fast plugs installed. Best thing ever. The injectors were re-newed 8 months ago,

My daughter has her 1985 300D at school in upstate NY, I'm in Missouri. She can't get her car started. I changed the pre-glow relay just befor she drove to NY, for the first time, in March. The initial problem was that the glow light never went off, the relay fixed that. A couple of weeks after she got to school she mentioned that the glow light stayed on after she started the car. Ignition on, light on, light off, start car, light back on always. This is different than the original problem when the light never went off during the pre-glow phase.