It seems that this switch is prone to frequent failure. My symptoms were a flaky transmission indicator on the dash and door locks that would not unlock when the transmission was put in park.
I got an estimate from the local GM dealer to replace this switch and they wanted over $600 bucks.
I am pasting a link concerning this switch
http://www.dieselplace.com/forum/sho...d.php?t=338137 here so you can better understand what it is and how to replace it.
I ordered a switch from my local Allison distributor for $80 (same switch & P/N) and will replace it myself. If you are not mechanically inclined, I am sure an independent shop can replace it in less than one hour.
BTW, GM charges $232 (list) for the same switch.