I do not find any test of theSIJ as valid.
I have suggested that the bigger picture is APAS, asymmetric pelvic angle syndrome. My article was published in the Alberta Pysiotherapy News of August 2002.
The test is to landmark the ASIS to ASIS, PSIS to PSIS, ASIS to PSIS(R&L) and then do hip abdcution directly lateral in the coronal plane. You will not see much if you do not do hip abduction. The suggested foot stances are 15 cm, 30 cm, 45 cm, 60 cm, 75 cm, 90 cm, etc. to the extent that the patient can perform.
If you want to do a simple check if the SIJ test that you are using is accurate and truly showing a "fixation" of the SIJ, simply landmark the PSIS to sacrum and have the patient do hip abduction directly lateral in the coronal plane. For every supposed "stuck" or "fixation of" the SIJ, the PSIS will move superior and lateral to the sacrum.
Hope this is helpful.
Neuromuscular