I have searched through a confirmed fix repair archive for this issue ( not google), and both those items are reported as confirmed fixes for this issue. I assume the other technicians are members of the same technicians network that I am. There is a TSB for this though it specifies issues with the instrument cluster as well as the DTC and staying in 3rd gear, which it sounds like you do have. In the TSB, the reported fix is the ignition switch which is where I would start. Here is the TSB
Electrical - MIL ON/A/T Stuck in 3rd Gear
Bulletin No.: 01-07-30-002C
Date: August 21, 2003
Subject: Malfunction Indicator Lamp (MIL) On, Transmission Stuck in Third Gear, Instrument Cluster Inoperative (Replace Ignition Switch)
Models: 1999-2000 Cadillac Escalade 2002-2003 Cadillac Escalade, Escalade EXT 2003 Cadillac ESV 1997-1999 Chevrolet Full-Size Pickup, Suburban, Tahoe (Old Style) 1997-2003 Chevrolet Blazer, Express, S-10 1999-2003 Chevrolet Silverado, Suburban, Tahoe (New Style) 1999-2003 Chevrolet Blazer RH Drive (Export) 2002-2003 Chevrolet Avalanche, TrailBlazer, TrailBlazer EXT 1997-1999 GMC Sierra, Suburban, Yukon (Old Style) 1997-2001 GMC Jimmy 1997-2003 GMC Savana, Sonoma 1999-2003 GMC Sierra, Yukon, Yukon XL (New Style) 2002-2003 GMC Envoy, Envoy XL 1997-2003 Oldsmobile Bravada 2002-2003 Oldsmobile Bravada with 4L60-E or 4L65-E Automatic Transmission (RPOs M30, M32)
This bulletin is being revised to update the model years and to add additional models. Please discard Corporate Bulletin Number 01-07-30-002B (Section 07 - Transmission/Transaxle).
Some customers may comment on one or more of the following conditions:
^ The Malfunction Indicator Lamp (MIL) is ON.
^ The transmission is stuck in third gear.
^ The instrument cluster is inoperative.
Upon investigation, one or all of the diagnostic trouble codes (DTCs) P0740, P0753, P0758,P0785, P1860 may be set.
The most likely cause of this condition may be a loss of power to the transmission on circuit 1020. This loss of power may be caused by an open in the ignition switch. This condition may also be the result of an incorrect installation of an aftermarket electronic device such as a remote starter or alarm system.
With the ignition switch in the RUN position, test for battery voltage at the appropriate fuse shown.
If battery voltage is present at the fuse, inspect circuit 1020 between the fuse and the transmission for possible opens. If battery voltage is not present at the fuse, replace the ignition switch using the Ignition Switch Replacement procedure in the Steering Wheel and Column sub-section of the applicable Service Manual. Refer to group number 2.188 of the parts catalog for part description and usage.
If the vehicle is equipped with an aftermarket electronic device and the DTCs are being set when the device is being used, verify that the appropriate fuse listed above is being supplied battery voltage during operation. If voltage is not present at this fuse, these DTCs will be set due to lack of power at the solenoids. This condition is most likely to occur with an incorrectly installed remote starting system. If this is the case, refer the customer to the installer of the system for corrections.