Maybe time for a quick door latch tutorial?
Hinged side doors of Ford E-Series van have various names; barn doors, 60/40, front/rear, etc etc. What we call them isn't an issue, distinguishing them from the sliding door option is though. This will address only the hinged side doors, in this instance I'll refer to them as 40% or 60%---following photo should be helpful:
The 40% door is most important because it preforms the bigger part of latching both doors to the body. There is both an upper and lower strike bolted to the body which the door shell mounted latches catch shown here:
The 60% door has just one latch, the 40% door holds the strike, shown here:
Because the 40% does most if its work with the door shell mounted latches, shown here:
Pay close attention to the latch shown in 40% Top above, especially the Torx drive flange attaching screws. The upper screw has worked itself loose, its lower counterpart will follow suit soon after. When/if either of these become loose the 40% door cannot firmly latch the door shell to the body and allow door seals to do their job.
Additionally the lowest latch of the 40% door can experience this same thing. Periodically check these fasteners---tighten if loose. I also recommend proper use of a thread sealant such as Loctite 290 (green) which is low strength, easily removable if necessary.
Typically the strikes do not become loose over time. Because the action between door latches and strikes is what seals the doors to the body carefully maintaining the latches themselves will maintain relatively tight doors.
In some cases door hinges or the strikes may need adjusting but that's a topic for another day and time.
I hope this is somewhat helpful, adds something to this thread.