According to the spinal surgeon, below, Tens machines are sometimes used to treat spondylolisthesis.
http://www.spine-surgeon.org/sas.htm
Check out the URL below positive reports from some patients who used them.
http://www.revolutionhealth.com/drugs-treatments/rating/electromedicine-tens-transcutaneous-electrical-nerve-stimulation-for-spondylolysis-and-spondylolisthesis
One the other hand surgery does seem to be the best option - if you can ever get it done. It is the treatment most widely recommended. Several websites mentioned that no studies have examined the efficacy of TENS in your disease. The last URL is from a website from a TENS manufacturer stating TENS can be effective.
http://www.spine-health.com/blog/surgery/back-surgery-best-patients-degenerative-spondylolisthesis
http://www.neurotech.co.uk/baxolve_conditions.php?bmrf=CB_spondylolisthesis
Good luck!