You might get enough clearance by dropping the front arm of the subframe. Undo the bolt that goes up into the body. The head of the bolt is visible just to the right of the red circle in the second picture. With that bolt out, pry down on the arm of the subframe. It should move a half foot or so.

If that's not enough, the subframe has to come down. It's attached at the other side the same way and at the differential. I think removing the differential rear mount is enough to drop the subframe. There are a couple of bolts at the top of the diff but I think they go into the subframe rather than the body.

At this point take a step back and determine whether it makes more sense to replace just the rusty trailing arm or perhaps the entire rear assembly out of a donor car. There aren't that many attachment points and you could end up with fresher parts for the same amount of effort.

Sixto
93 300SD 3.0 - comatose