I know Korn made a tool for this job, but I don't have it.
I do have a number of wasted drive shafts. Newer, older, doesn't matter, its the wasted part that's important. Cut the bell off of the old shaft. I cut mine flush with the flat inner surface. Then weld the open ended bell to a flat bar (1-1/2 x 12 x 1/8") that is radiused to roughly fit the upper diameter. Pretty welds are not something I know how to make....
Now a 24mm socket will fit neatly into the hole. Open the locknut while holding the input shaft with the new tool. Highly effective
The notched seal retainer ring you see in the lower picture opened easily on my drive with a sharp blow to blunt end drift. Seemed like a pretty safe move and it worked. I won't be able to measure the torque getting it back on, but I will get it on quite a bit tighter than it was coming off.
BTW, if anyone ever needs to borrow the new tool, drop me a line.