If you've tried the McKenzie extensions with little to no benefit (I did, for several years) maybe you don't have a slipped disk. There's at least one condition that doesn't respond to back extensions at all: Alkylosing Spondilitis. It's kind of like rheumatoid arthritis, but it targets your SI joints (and often other joints as well). One of the other diagnostic features is that your pain will sometimes switch sides, for no apparent reason. About 10% of back pain cases are due to Alkylosing Spondilitis. A more offical diagnosis depends on checking your blood for HLA-B27, though this is not conclusive. An MRI or, x-rays can also help make the diagnosis, though X-rays usually only show disease progression after 4 or more years after pain onset.
AS treatments include NSAIDS and Tumor necrosis factor-alpha supressors.