Tension comes from trying to control (aka trying to stop or minimize stuttering in some way). Part of this is conscious and part of embedded from years of reinforced behavior. The good news is tension can be worked on with the help of a good speech therapist. You tense as a result of avoidance because you don't want to stutter/sound weird/feel embarrassed. I would find safe people or places where you can very slowly work on not tensing and letting the stutter come out however crazy or easily as it wants.