99 Silverado
 

These are the symptoms. Tow/Haul wont work. Speedometer started quitting and going to 0mph then truck would not have 2/3 gear(auto) Restart the truck all is well for a bit, just no tow/haul. All was working as of 2 weeks ago just fine. No ses only the p0420/p0430 for cats. Any ideas?

1999 Siverado 471 4wd 200k

Change out the VHS(vehicle speed sensor). It senses the speed of the transmission for your speedometer and also will affect shifting. It is located on the rear housing near the tail where the driveshaft goes into the housing, on the passenger side. It is held in with 1 bolt(5/16 or 8mm). The part is around 24 dollars but you need to count how many bolts are holding up the transmission pan to get the right part. Its either 16 or 17 bolts. Try that first and if it doesn't fix it, then it might be your ignition switch.