Chalk is what you used to write on the sidewalks with. CHOCKS are what you put by your tires to limit movement.
OP, You have your stab jacks down and X-Chocks and are still getting movement? Are you sure your X-Chocks are tight enough? Go outside and watch your wheels and have your wife try to shake the trailer. If you still see the wheels rocking back and forth, your X-Chocks aren't tight enough.
If your stabs are tight and your X-Chocks are tight, you are pretty close to being out of options. The JT Strongarms are one option. The only other thing I can think is to add another set of stab jacks in between or directly in front of the wheels to take some of the trailer suspension out of the equation.
Jacks under the slide are not a good idea. If something should happen, like a flat tire, causing the main body of the trailer to settle and your slide isn't allowed to settle with the trailer, you will have some SERIOUS problems.
2015 Ram CTD
2015 Jayco 29QBS