To everyone reading this thanks for your time. I have a 2001 silverado Z-71 and recently stumbled upon a huge problem. Occasionaly on the interstate my speedometer cut out and the truck downshifted, speedometer would kick back on rather quickly and transmission would re adjust. This happened 2 or 3 times. Less then a week later the speedo began to cut in and out and the transmission would shift according the speedometers reading. After some quick help from google i determined it was the vss(s). I replaced the 2 on the rear of the transfer case and the one on the front as well. The two rear ones had a groove worn into them. Within the first few minutes of my testdrive the same problem began to occur again. I nursed the truck back home, pulled the sensors and the new ones (2 rear) already had the same "groove" worn into them. Any suggestions greatly appreciated, it looks to me like something in the transfer case which is beyond my mechanical ability.