You can meditate standing up. There is a static tai chi position where the knees are slightly bent, the arms bent slightly at the elbows and away from the torso, and the chin lowered to stretch the spine, feet facing forward at shoulder width. It is surprisingly free of strain. If sitting, just put a hard pillow under the tailbone to keep the back straight. You can meditate in a chair with backrest and a pillow behind the lower back if there are problems. If you can resist falling asleep, lying down works, at least for a portion of your meditation time. I meditate in bed, and even tone mantras in bed half an hour to an hour before getting up. It used to be the only way I could do mantras and not get tense. Over time, I learned to do them in other postures, or while walking, washing dishes etc. Karma yoga is based on a meditative state (a light one, but still effective) while you are going about your business. Maybe not while driving, but waiting in line or any instance where you can be impatient can certainly work.