4R55E reverse problems

I need some advice on my transmission, I have a Ford Explorer '97 with a 4R55e transmission. It runs great in all forward gears, park works but if I select reverse it still wants to go forward. I took it to a tran shop who wanted a fortune to fix it saying the reverse band was stuck. Well I am pritty handy around cars but a novice on automatic transmissions. I wanted to know if that sound right or if it was some sort of servo or select problem. I have removed the transmission and bought the ATSG manual and am very keen to find the fault. Please let me know that I am not in over my head.
Rgds
Brian