Clean fluid in the transmission is a great starting point. Does your truck have a throttle cable or is it DBW? The trans will shift odd when a TPS is going out.

Did you get a new tune sometime recently? Maybe the TM wasn't removed on the last tune. It seems all your transmission issues came all at once. You have a Corvette 1-2 servo installed, right? Does the transmission slip when it goes for second or does the engine power puke during the shift? Do you have a SES lights on?