Seeing true upslips...
They are not too common, the reason being is that they are not physiologically movements of the body and typically only occur as a result of a trauma. The physiologically motions of the pelvis include the anterior/posterior rotations and the in/outflares. These are classified as physiological because they occur naturally, i.e. during the gait cycle. The non-physiological movements (the upslips and downslips) occur secondary to a trauma, i.e. a fall down the stairs, on ice, etc. The shearing of the pelvis on the sacrum would occur because of the force from the fall. Think about it; it's going to take a lot of force to shear theSIJ.
As sharileedahl has so aptly put it; there are other factors that can 'fake' an upslip. The most common being myofascial tension through the QL, psoas, or lumbar spine unilaterally. Of course a proper assessment needs to be made in order to properly diagnosis and treat the lumbopelvic complex. Chances are that if there has never been a significant trauma to the pelvis (i.e. a fall) then it would be safe to say that there is no upslip.
As far as treating the SIJs manually, I have seen miracles happen. I will tell you a quick story regarding a neurosurgeon who came to us for treatment. He had a significant disc protrusion verified viaMRI, had gone to many of his colleagues for opinions, all of whom had told him he needed surgery. Being fully aware of the implications of the proposed surgery he wanted to exhaust all other options before going under the knife. Fortunately for us, we got to work together. We began by manually correcting his pelvic, sacral, and lumbar spine mechanics. Things began to turn around immediately. We would integrate this with minimal and basic exercises, more along the lines of re-education, i.e. pelvic tilts. It took some time but now he is pain free and has better motion than before.
In short, there is a lot to be said of correcting a persons structure in order to positively influence their function.