2001 Ford Explorer Does my Transmission Need to be Replaced

I have a 2001 Ford Explorer Sport with automatic transmission that I purchased used 6months ago, had a complete tune up, SUV performs great. Last few weeks, my SUV switches into O/D with the flashing light illuminated on my Dash and stays there until I turn engine off. Goes away when the engine is restarted. Feels like the car is dragging or downshifting, usually happens about 8 miles into a drive but could be less. Also when I pull out of driveway notice engine reveving up when I accelerate.
Took car to local mechanic, he ran diagnostic test, says that SUV is downshifting automatically from 4th to 3rd gear. Says PO733 and PO734 are Incorrect Gear Selection. Advises probably needs new transmission, cost can varies until they get into working on it because of technology/computer sensors and they find exact root of problem, high estimate of $2,7000, low of $2,000.
Should I take to Ford Dealer for another diagnostic test? Really don't want to put out another 100 bucks to tell me what I already know. My mechanic has excellent reputation, straight shooter, just want to be sure or done good research before I lay out a couple grand. Advice?