Mileage on this trans if known?
Any recent repair work?
You can not place a 700r4 non electronic trans into this truck easy... the ECM looks for feedback from the 4L60E trans. No feedback = trouble codes set.
I doubt you have a shift solenoid problem since there are only two shift solenoids in this trans that perform double duty for shifting 1 - 2 - 3 - 4.
I would suspect a worn out 2-4 band with a high mileage trans
Dont worry how old you are... it is the mileage on your body that adds up.