I have the exact same crack in the exact same door on my '95 DCM. I noticed it when I was trying to improve the body panel alignment (the factory didn't do so very good of a job on these cars). In my case I couldn't get the door to close far enough. I ended up machining the striker plate and used some offset bushings on the holes.
The strange thing about this crack is that the striker backing plate pinches the metal in that area, and the metal shouldn't have been stressed enough to crack. The good news is that I have kept a close eye on mine for 5 years now, and it hasn't spread. That said, I rarely open the back doors on that car, it only gets driven on sunny days, etc. Basically it's a pampered garage queen.
With careful masking you should be able to get the crack welded up. You'd need to get access to the back side, which may be tough. I think a bracket loosely holding the striker backing plate is spot welded to the back side. At least that's what it looks like is happening once the striker is removed. That area is tight and somewhat hard to get to. You may be able to just weld from the door jamb side and very carefully grind the metal flat. You may want to drill the ends of the crack before welding to keep it from spreading any further.