Is cracking your spine after back surgery fine?

No! Once healed!: No-you are simply doing the equivalent of "cracking your knuckles"- it feels good or it just becomes a habit. By "cracking" or mobilizing the small joints of your spine(facet joints) it causes a temporary reflexive relaxation of the surrounding muscles. This is what an osteopath or chiropractor will attempt to achieve with manipulation. You are doing your own! you are not causing any arthritis.

Be careful: Cracking the spine after surgery should be avoided. Would ask your surgeon too. Timing for resuming strenuous activity after surgery depends on the type of surgery done.