pic: Unobtainium 6wd - Cantilevered, Dead-Axle, Slot-Tensioned Drivetrain

[Tristan Lall]

You might want to try heavy-duty all-metal locknuts (the ones with the elastically-deformable bore), or high-strength steel nuts and threadlocker. Torque them to the limit specified by the manufacturer. Given the shoulder bolts, you'll probably need an unusually-strong Allen wrench to make this happen. That's the only way I could see this holding tightly enough in a slotted hole.

Consider http://www.mcmaster.com/#92501A030 or http://www.mcmaster.com/#93591A200, for example.