If it's like other cars I've worked on, the hinge pins wear. This causes the top hinge to "spread out" a little, causing the door to sag at the latch end. I'll take a wag that any 2 door will have longer, heavier doors thus more leverage on the hinge pins and more wear.

Cures:

1. new hinge
2. new hinge pin or bushing (typical GM)
3. if it's the pin and not the bushing, just rotate the pin in the hinge so that new, unworn surfaces come into play. You may need to pull the pin to rotate it.
4. possibly, just move the hinge (door alignment) to take up the wear. Once it's started though, the wear will continue.

HTH,

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Norm in NJ
Next oil change at 230,000miles