I have the same problem on my '86 LTD; I have decided to back the doors with a thin, 1/8", piece of masonite or such. You will need to rout out the existing grove so the 1/8" piece will fit down in the groove or you can cut the board the exact size as the outside frame, shim the center panel so that it touches the front rails and screw the center panel to the board. Staple, screw or nail the board to the rails. This will do two things: center the panel and reinforce the rails. You will need to re-install the hinges over the board and bevel the board edges and stain them to match the rails. Don't glue the panels because the wood will split under some weather conditions. Have fun.