Here is a possible theory. Yes, I realize it probably did NOT happen this way due to JD's fear of him, but what IF FD had contacted JD and/or previously made arrangements, or watched her drop kids off at school and followed her until she stopped at park. But some how, JD met up with FD at the park. Say for example, he then proceeded to tell her a bunch 0f lies like he wanted to try to get along through this divorce, agree to pay GF back some money he borrowed, or whatever his story may have been, and JD got in with him to talk, or he forced her to get in with him, and then he drove to her house, killed her in the garage, put her back in his vehicle and then got rid of the body, which meant then that her Suburban stayed at the park right where she left it??
(Yeah I know, as I mentioned, it probably didn't happen this way since she feared him, unless he forced her into his vehicle).