I would definitely just try the walking (not running, too jarring) and skip the crunches, see if it helps. Maybe you will have to walk at a more moderate pace, if you are speed walking? At least with walking you are upright, but the speed might be a factor too?
I read an article that some people pre-treat before exercise by taking an H2 or PPI. I have no personal experience with this though. Maybe your GI Dr. can advise you on that?
I'm relatively newly diagnosed so I'm just basing these things on articles I have read. It appears exercise induced GERD can be a problem for many.