Broken shock mounting bolt...

You can replace them... There's a nut on the back that you can take off from the inside as ch1ll has stated and Mancini has new replacement ones if you can't find a good used one...

Mancini Racing Lower Rear Shock Stud
Honestly what? replace both... metallurgy does have cycling and fatigue failures....
And is the O.P running S.S .springs ?