Thought a little more about this and came up with a few ideas for you. The pic below is poorly done, but all I was able to do with the lousy software on this computer. Basically, I took the double upright you have going to the rockshaft, and laid it down, back to the disks. Then install two angle iron bars from near the outside of the disks, back to the drawbar. You may want to do a single bar across the top of the drawbar, with holes to mount it to the drawbar, then attach all 3 of these bars to that single bar, to make it easier to hitch up. Doing this will keep the disks ridgid behind the tractor, and will keep it from swinging.
Then, I'd use a chain down to the drawbar, and a second chain back to the back of the disks. You may need a turnbuckle on the rear chain, as this will now set the angle of the disks and the turnbuckle will let you have some adjustment in the length. I would leave the drawbar unpinned, and use it as the lower rockshaft, so you can raise/lower the disks with the upper rockshaft.
Like I said, just my thoughts. Here's my lousy markup of your picture.