Goin down hill @ 40-50 mph , upon stepping my brakes to slow down my transmission drops gear making a loud noise and as if my my gears goin to lock up

Hello, there could be input information causing the transmission to do what it's doing, or it could be a transmission concern. Hands on testing by a skilled and experience technician with proper test equipment is needed. Use this link to find some shops near you that can help with your concern: www.repairpal.com/repair-shops