Let us correct. Things, there is difference between sacrum and coccyex, the sacrum on the top of coccyex, no you can not crack them.
You can. Crack your sacro-iliac joint and lower lumbar facet joints like you crack your knuckles, however it's not down at the tip where the coccyx is, but higher and off to either right or left side.
My lower back pops, almost like when you crack your knuckles. But its one of my lumbar vertebrae. I have lower back pain but not when it pops?
Ok. This is ok and you are doing your own spinal manipulation like cracking your knuckles. You are most likely "cracking" your facet joints which stretches their joint coverings or capsules which causes a reflex relaxation of the surrounding back muscles giving you relief. There is no harm in you doing this. The popping sensation is the joint surfaces separating which are "wet".