2005 Suburban speedometer erratic & transmission shifts

[Aboonah]

Our 2005 Suburban suddenly had the speedometer become extremely erratic, jumping from 0 up and down to 50 or 60 and everywhere in between. At the same time the transmission began to seem to try to shift to keep up with the speedometer changes.

I was able to drive it in 2nd gear by watching the tack and staying just below 2000 rpm.

Mechanic pulled the cluster and had it rebuilt. No change.

So the mechanic said he got a generic transmission code and sent the vehicle to a transmission guy who checked the tranny and said it was cherry...no problems there or in the valve body.

Now our mechanic gets a code 292 from various sensors indicating no communication with the PCM. He says it appears none of the sensors are replying to the PCM, but the PCM tests good. But only the ABS system dash indicator lights. Also no other systems seem to be in trouble.

Related: yes, no, maybe?

Any advice appreciated.