The problem you’ll have is at that age and mileage, almost every shop is going to want to just replace it with a reman transmission. Even if it’s something minor, there’s a lot of potential for a subsequent failure for which they don’t want the liability or hassle.
If you don’t have a trusted private shop then you could start at a GM dealer and ask them first to just diagnose it. It’s possible they’ll scan it and give you a diagnosis other than total replacement if you’re explicit on asking what the failure is and what your options are. Paying them to drop the pan and look inside is a worthwhile expenditure, as a look at the debris in the sump is helpful.
If you do not have the maintenance history it diminishes the opportunity to get meaningful information from examination of the pan, because if it’s the original fluid that’s a lifetime accumulation, vs if it was serviced 20k miles ago.
