2007 5R55S with Code PO745 Help!

[newmaneh]

Hi! This is my first post to the forums but I've referred to this site many times so I know that most of you are experts and I'm hoping you can shed some light on my situation. I have a 2007 Explorer, 5R55S transmission, and noticed revving around 20 +/- mph and sometimes thumping into gear. Today a mechanic told me that I have no 3rd gear ("It's gone, goes from 2nd to 4th") and it's getting code PO745. He immediately informed me that the transmission needs rebuilt soon because I'm on borrowed time, for $2500. I know women don't tend to question these things, but I'm in IT and I troubleshoot for a living - so I came here. Several threads that I've been reading here mention replacing the EPC solenoid first. However those threads aren't identical situations to mine - some are missing 2nd gear, and some also have additional codes. (The mechanic only informed me of PO745.) Any advice? Any feedback would be appreciated! Thank you in advance.