The other day drove the 200Chevy Blazer 4X4 to work and it was fine. Went to leave and the speedometer didn't work and the transmission won't shift out of 1st. Ran codes and said VSS was bad. Replaced it and didn't change a thing - now what? We have the button for 4X4 that you can push and it comes on as needed rather that shifting into full time 4X4 - does that make a difference on how many VSS there are?

 

Mine has three. Change the OTHER rear one.
- just a guess.

 

There are two speed sensors on the rear of a NV236 transfer case. Only one of them is the VSS. Swap the old one you replaced with the other one in there and see if that cures the problem. If not, run a search for "nv236+speed+sensor" and you should find the relevant tests that can be performed to get down to the cause.