That is the yoke nut. I believe it is 1 1/4" for the big shaft V6 D18.
If for some reason that is too large, try a 1 1/8"
You will want to use an air gun if it is accessible. Otherwise, you will need to lock up the drivetrain and attempt to remove it with a breaker bar.
Before you remove the nut, check the vertical movement of the yoke. If there is slop, be sure to reset the end play by removing a shim or two in order to tighten up the output bearings before you reassemble the parking brake.
I would plan to replace the rear output seal while I was in there.