Bottom freezer door sagging

The freezer door is sagging, and there is a noticeable gap at the top right of the freezer. It keeps icing up inside near the gap. Hinges are on the left. Now I've noticed that liquids on the top shelf of the refrigerator are freezing as well. We have always had a problem with ice formation in odd places inside the freezer; only during the last few days have I seen a visible gap in the upper right corner. If we lift up on the upper part of the door, it shifts around on the hinge. It seems like maybe a spacer of some type would help, but I'm not sure how to install one or what size to buy.

Thanks for any help you can offer.