Is it dangerous to crack your back if you've had a spinal fusion?

Probably: I do not recommend cracking your back, or having manipulative treatments, particularly after a spinal fusion. A fusion permanently stiffens the segment(s) where there was pain generating pathology. There is always the possibility of a breakdown in the integrity of the adjacent spinal segments, and such techniques would cause more stress at these areas of future risk.

No: No-it is not dangerous, harmful or risky as long as you are doing it yourself.