you need to take out the rubber bumpstop itself to access the bolt that hold the cup to the bottom of the spring post. ive always had trouble getting the rubber out. if you still have the coil in, ive found it works well to use a pair of channel locks and reach through the coil and grab the rubber bumpstop. then using the coil as a fulcrum, you have leverage to pop it out. then you can access the 15mm
edit: or you might have luck using a large flat head screwdriver vertical to pry it out
i think some people grind the edge of the cup so they dont have to remove them to clear the isolators or coil spacers.
'98 ZJ on 37s and tons
'97 Cummins 2500