If this problem is a result of the latch being worn out, and you are
feeling cheap I can offer an alternative solution that I have used.

I was working on a van with this problem and realized that the Magic
Opening was caused by wear in the latch.  Wear had pretty much dulled,
rounded and changed the angle on the bottom side of the crab claw
looking thing (sorry about the bad terminology),, the part of the
latch that rotates and locks into position while grabbing the
adjustable post on the rear side of the door frame.  I used a dremel
tool with a small metal-grinding head and re-angled the inside of the
bottom part in the "claw".

I know this is a poor explanation but if you want to spare yourself
the hassle and expense of buying, installing and adjusting a new latch
mechanism you may want to explore the latch to see if you notice the
rounding off wear pattern that could be causing your problem.  Once I
had located the problem it only took one or two minutes of careful
grinding to fix the problem, and yes the door still stays closed no
matter how bumpy the road or how hard you slam it.

Jonathan Poole