||They might do it, but it's not going to release ecstasy or serotonin into their systems. Ecstasy is not stored in spinal fluid, nor is spinal fluid released into the blood stream when the back is cracked.|
None of this means that having ones back cracked after taking MDMA can't lead to an increase in effects. While I'm not aware of any physical mechanism which would cause such an increase, the placebo effect is very strong. If someone believes that back cracking will increase effects...it just might.
There's a lot of myths, ceremony and rituals around the use of psychoactives, this sounds like one of them.