I had a similar problem on my 2006 and 2007 Ram 1500 trucks. I had the dealer tech fix it. Before, The warning bell would sound when I didn't have the belt on. I run around on the construction site getting in and out several times a day. The doors would lock upon reaching 15 mph in forward motion. That feature still works. Also since the fix, BOTH doors unlock when I pull the drivers door, unlike before. If you get frequent oil changes at the dealer, and they want to keep your biz, ask them to fix the problem. It takes them 2 minutes with their code/diagnostic tool.