I had both my doors done, (new sheet metal, and new paint) in Marinette, WI, for$1000. I'm very happy with everything they did.
In talking to the guy that did the work, he said he had done this rust repair on at least 30 or 40 of the 2nd gen. Dodges, mostly back in the 1990's, authorized and paid for by Dodge Inc.
According to him, it was not so much a road salt problem, as it was poor design. He said among other things, the water drain holes were placed too high. Consequently almost all of these repairs were due to rust caused by an inch or so of standing water, in the bottom of the doors.
After the repair, he drilled two drain holes in the bottom of ea. door, and said if it ever rusted again, he would fix it for free.
Good Luck, Ray