Your SI Joint, Sacroilliac Joint, is what connects your spine to your pelvis. It has only a small range of motion compared to other joints in the body. The SI joint can be locked up, or too lax and unstable. The SI joints help to support the weight of the upper body, and absorb shock between the upper body and lower body. This helps reduce the amount of pressure on the spine. When they aren't doing their job, this can present as pain in the hip, buttocks, or lower back, and can sometimes have radiating pain down the legs.
In order to work on chronic SI dysfunction, please consider the roles of fascia, mobility, stability, and effective core and glute strength.
You will need a foam roller, a small therapeutic ball or tennis ball, and a looped resistance band.
Enjoy, and Namasté.