Automatic Transmission problems of the 2007 Chrysler Aspen

I have only had my truck for a year and four months. There has been at least 4 times in the last 2 months when I put my truck into reverse the transmission defaults to neutral and the box around the "r" would disappear. The other day I was putting my truck in the garage and I put the truck in reverse, I looked back and took my foot off the brake and pressed the gas pedal, being that my driveway is on a slope my truck started coasting down the hill fast. I turned around to put my foot on the brake and just in time from hitting my husband. My heart fell to my stomach. I asked him to look at the dashboard so he could see that the truck was in reverse. The box that outlines the letters to show which gear you're in was around the letter "r" for reverse this time.