Thank you for using just answer!
What is happening is there is either a problem that is storing a code in the computer and it is defaulting to second gear or as in a copy of a similar problem posted below they have a problem with the ignition switches dropping voltage which can cause a no shift condition. The first thing you need to do is have it checked to see if there is a code stored. If there is a code stored then let me know and we can follow up with that. If you have a voltmeter you can check the fuse to see if the voltage is the same as the rest of the fuses. If it is lower as in the post it will also cause it not to shift.
The transmission on this vehicle will intermittently defaultto second gear. I've had other GM trucks do this with ign.switch problems i.e. low voltage to term. 20 in fuse panel.However, when this problem occurs the voltage is normal. Italso doesn't exhibit the speedometer/tachometer problemsnormally associated with ign.switch problems on thesevehicles. I did note that when the problem occurs the securitylight comes on with no starting problems and I lose scan tooldata stream. Any help would be greatly appreciated!____________________________ FIX ___________________________The problem turned out to be the ignition switch, even thoughthe symptoms were a little different than the ones I'd had inthe past. Voltage drop to 10v on fuse #20 when the fault waspresent. Thanks for the help!