Please help - Shift solenoid error code (p0756) [5EAT]

[Luckyy7]

My car has an error code : "shift solenoid (B) performance or stuck off"
When it hits third gear the sport light starts flashing and it stops shifting, then when I take off from a red light I hear a mechanical noise coming form under the car, mind you the car takes off very slowly and with high rpm's (guessing cause it's stuck in maybe 4th gear or something). There was a gearbox leak that I was unaware of, fixed the leak and filled oil, what should I do to fix this problem? Anyone encountered it before? Please help