The 15-22 has the spring and detent for the rear take down spring held in place by the grip. On AR15 forged lower receives the spring and detent are not in the same place as the 15-22 and are held in by the receiver extension end plate.
Most grips will work on the 15-22. Any grip that you buy will not have the second hole since they are made primarily for the AR15. You can either drill a hole into the grip that matches the original A2 style grip. Or trim the spring a little bit, but do not trim it flush to the lower, leave a bit sticking out so that there will be spring pressure applied to the detent.