So I have a 2006 sierra with a 4L80e trans. I blew up the original trans installed a used one and never had 3 or 4th gear so I bought a rebuilt one and the same thing. I have replaced the VSS and the main external harness to the trans. The first trans had a code for excessive slippage and now the last two have had codes for solenoid B stuck open don't think it's the trans it's self leaning towards electrical any helped would be great the code is P0757. Thanks Also the battery goes dead every once and awhile

 

Shift solenoids can actually be a pretty big problem with those trannys. Did you try replacing the solenoid on either of the used transmissions that you tried?

And as for the battery, they do wear out after a few years. Its probably time to replace it.