2004 chevy silverado 3500 tranny stuck in second gear. no codes and no _ under P R D 3 2 1

The "2nd gear" default problem you are experiencing is referred to as "limp home mode" The transmission will continue to run in 2nd gear so you are not stranded and are able to get to a service area or home.
The reason the transmission is stuck in 2nd gear is due to an ecu failure or wiring problem that has occurred. The transmission control module can no longer control the transmission or does not know what state the transmission is in.
Unfortunately, you will have to have the transmission diagnosed and serviced. Most likely it is not a serious problem and should be able to be diagnosed an fixed within a short period of time.
Good luck.

Engage the Parking Brake.
Hopefully the Shift Cable has not broken=snapped. If it hasn't then:
The shift cable most likely has become loose at the shift lever connection located on the transmission. Go to the transmission and inspect the shift cable to shift lever connection. The shift lever goes into the transmission. IF it is loose/not tight enough, you should be able to shift the lever by hand without the shift cable moving OR you should be able to advance/retrac the cable by hand without moving the shift lever.
Disconnect the shift cable to shift lever connections.
Move the Shift Lever into the Park Indent position. Pull on the Cable and the __ should reappear in the PRND321 window. With a Partner helping: pull the cable until the __ is under the P. Reconnect the shift cable to the shift lever and retighten the connections down to approximately 100 in-lbs.
This should take care of the problem. Test the engine start-up by starting the vehicle in Park and also Neutral, as these are the only two positions it should start in.
Let me know if this helped, or if you have any additional information or questions. Feel free to contact me at FixYa.com!