Where is the output speed sensor located?

P0722 is the code for output speed sensor, no signal. It can mean the wire to the sensor is bad, the sensor itself is open or it has shorted to ground or voltage. On a scan tool it would show up as 0 mph regardless of the speed of the vehicle. On that transmission the input and output speed sensors use the same harness assembly that is located on top of the valve body inside the transmission. As you are not showing a code on the input sensor as well the 8-volt supply is okay, and the issue is the sensor or it's signal wire to the trans control module. The control module is also located inside the transmission and the sensors plug into it. To replace them you jack up the truck, drain the transmission fluid, remove the transmission pan, then the filter and remove the lower and upper valve body assembly. Then you can get to the sensor to change it out. Then reverse the process, using new fluid seals between the valve body and transmission housing then install a new filter and seal the pan and install new fluid. Another common problem with that transmission that can also set that code is the internal wiring harness will fail and leak fluid out into the main connection on the side of the transmission. If that is the case, then you replace the entire harness assembly and the sensors inside the transmission. So yes, they are correct it is inside the transmission.

That would depend on the tools you have available as you will need a scan tool to verify that there is no signal and that it returns when you replace the sensor. Then once complete you need the scan tool and a link to GM to be able to reprogram the control module to the new parts. Looking around I don't see any videos on the process. If you want to do it yourself, I would first verify which transmission you have there were both 6 and 8 speed automatics used that year. The process with both is pretty much the same thing, both require the fluid, filter and valve body be removed. Then you need the scan tool and a clean place to work. The first two images are the 6L80 or 90 speed sensor locations and the procedure in the book. Then next are for the 8-speed transmission, for it you require a special DT-48285 - Valve Body Torx Plus Socket to get the bolts out. You will also need the new bolts to replace them with. You remove the old sensors and replace them. Once that is complete you bolt the valve body into place and install the filter and pan and refill. Then you follow the reprogramming steps. The mechanical part isn't that hard. It's a bit more involved than a fluid change because you will be removing the main control assembly from the transmission and any issues with that process can result in a non-functional transmission. As such it's one of the items that unless you have experience working on transmissions, I would have a shop do. That way it would also have a warranty as well as if something did go wrong.

Go back over the work and be sure there isn't any harness damage. Then use a scan tool to check on the solenoids and signals. Then do a reset on the system and see if you get the transmission to come back online. As it worked prior to this it should be something simple like a poor connection or possibly pressure leakage. If it's the 8 speed the relearn process has to be done if you changed any parts.

Some shops can and dealerships can. There are also some mobile techs that do it as well. All depends on the area you are in.

That is the manual detent. Installed wrong it shouldn't cause no motion as it simply acts to stop the manual lever from moving once you shift. However, the manual selector and the position sensor could cause no shift. That is the sliding switch on the valve body that fits over the pin on the manual shift lever inside the transmission.

Just checking to see if you had any luck with your truck. Haven't heard either way.

Sorry about that. Metal shavings definitely isn't a good thing in a transmission. Thanks for the follow up though.