I wash my back with my usual body wash and I find that it's not a problem. Sometimes back pimples are caused by the hair products you use, you put the product in your hair, it rubs on the back of your neck, tops of your shoulders and your back, and causes pimples, so it could be simple as that.
To put moisturiser on my back I put a line of it on the backs of my hands, and I find it's a much easier as there's less twisting involved. Another way it to sew a piece of velcro to a makeup sponge, stick the other piece to a ruler, then stick them together, put some moisturiser on the sponger and you've got plenty of reach with your ruler. And the reaon for the velcro is so you can take the sponge off and wash it. I know it sounds weird, but it does work.