Can just the shift solenoid be replaced in the transmission & at what fair cost?

There are no less that 8 solenoid specific codes on my generic DTC list. Best bet would be to go into your local servicing shop (recommend a transmission specialist) and have a proper diagnosis performed to determine the specific solenoid circuit that the computer is detecting fault with. Additionally if a solenoid is functioning properly and the commanded clutchpac is worn - it can generate a solenoid fault code. And in California, the check engine light is an instant failure. The computer detects faults that increase emission levels and so the computer generates a code.