Get the car scanned yourself at autozone. Forget the damn mechanic for now. The light is only on when the car has detected the code and is doing something to compensate. Once you turn the car off and turn it back on the light will not go on until the problem has been detected again. Now, just because the light is not on does not mean that there is no problem or that there wasn't a problem, it just means that since the car was last started that the problem has not happened again (or in reality the problem has not been detected by the computer).
The nice thing though is that when a code is set it is stored in memory for later recovery. However after the car has been cycled through a number of running cycles and the code has not been set again the car will erase the code. So if you wait too long after the code sets it will eventually be deleted from the memory if the code does not reoccur. I'm not sure how many engine on/off cycles this is but let’s just say 30 for example. So if you run the car 30 times and the code no longer sets then it will be erased automatically.
I would get it scanned now and see what comes up, if nothing comes up then wait until the light comes on again and get to autozone as soon as possible so that it doesn't get erased.
Also the codes for 96 and up look something like P1234. The letter tells the area of the problem, P for example is powertrain, B would be something like body control module and there would be a letter for ABS etc. Then the four digit number tells the rest.
A code like 94 would be set on a 95 Aurora or pre OBDII system. You really don't need to worry to much about the logistics of the code though since the scanner should give you a word description (and the code itself). If the scanner doesn't give a word description then you can attempt to get the description at
www.obdii.com.
I'm not sure what the code for a shift A/B solenoid problem would be on a 96 and up car though...but that doesn't matter since all that really matters is the code that is retrieved by the code reader. If the reader says there are no codes then keep driving until it gives you one.The think to remember is to let the car do the work and detect the problem itself...no sense in trying to guess the problem. Car self diagnostics are fairly comprehensive these days.
Anyways there is a TSB (Technical Service Bulletin) for the shift solenoid problem. I have a copy of the TSB here
http://ghutchin.dyndns.org/pics/ShiftSolenoidTSB.htm
The GM part number for the kit is 24211355 and this will only be available from a GM dealership or online GM parts supplier such as
www.gmpartsdirect.com
If you do get one of the shift solenoid codes then take it to a transmission shop or a dealership. At a trans shop this job should cost on the order of $300 or so. (I believe this is including parts) Do not have a regular mechanic do this type of work, it will just end u pcosting more.
And in general I never find it good idea to tell a mechanic what you think, this just offends them ( which makes sense since you are after all coming to them for help and when they offer it the last thing they want to hear is that you think they are wrong). Its best to go armed knowing what the problem is, wait for them to confirm it and if what they tell you does not agree with what your own investigation has told you go somewhere else.
Get the car scanned at autozone...it is free
Good luck
-Glenn
P.S. I'm not a mechanic, I don't particullary care for them but at the very least I respect their decision and allow them to do their thing (within reason) when I have no idea what the hell is going on (although that doesn't happen that often).