I was looking at the lock mechanism the other day.  There isn't an adjustment
on the mechanism itself (maybe on the lower part -the rod that sticks up).
The mechanism has two "steps".  Anyone have any idea why it has two steps
instead of just one?  My only guess is that incase the final step gets worn
out the first one might hold the door shut or maybe if you shut the door
without any force the first one would catch (though the door would be a
little loose).
The only other thing you might be having a problem with is either your rear
door seal (maybe it has come loose in some spot and is jamming between the
door and the body) or the rubber spacers in the two, bottom corners of the
door.  These could be adjusted out too far (how this could happen I don't
know) and not allow the door to move close enough to the latch without extra
effort.
Let me know what you discover.
Ken Wilford
Van-Again
John 3:16
PS If I knew everything I would have won several lotteries and be emailing
you from my own personal island somewhere in the pacific. :)