I have A 4x4 2000 Blazer 4.3 and 4l60e. My trans is not shifting on its own. I can shift to 2nd by manual shifting. Speedometer not working. I have replaced both speed sensors. I am not getting +power to one of the sensor plugs. Had codes checked the only codes that showed up are o2 sensor codes. I have had blower motor and head lights stay on after key off A few times making me wonder if ignition switch could be bad. but not sure If that could be related to the no shift problem. Also wandering if ecm or wiring could be the problem but not sure. The reason I suspect ecm is I think It should throw A code If speed sensor is not working correctly. It randomly started working one day for about 10 miles then quit again.

 

Check your fuses.

 

I checked the fuses today. All of them were good.

 

Looks like wiring to me.
Do a visual inspection of the complete harness from the VSS on the transmission to the ECU (TCCM/PCM). At the same time check the wiring in the engine bay.

In addition as previously said, check all fuses.

Check for broken wires, chafing, bad repairs and "rats-nests" of wiring where somebody has fumbled with the wiring.