I fixed this on an 85 that I was working on a few years ago.  The repair was
easy and the door still stays latched.

I posted an explanation a few years ago that is probably better but I will
briefly describe what I did here.

In summary I took a dremel tool with a small pointed grinder bit and changed
the shape of the rotating latch part on the rear edge of the sliding door.
The part that I modified sort of looks like a crab claw and one of the two
the "teeth" had rounded off.  It was the "tooth" nearest the van that hooks
under the post on the door jamb.  It seems like the deformity was obvious
when you looked at it but I don't remember it that well.  Basically I
changed the angle of that tooth and played with it until it worked.

Maybe that will help someone out?

Jonathan Poole
'82 ac westy