truck will go in reverse and drive but when you start down the road and when you get up speed and it is suppose to change on its own it wont change out of low . it is an automatic.
have the computer scanned on it first ,because its probably went in to limp mode on the transmission,its like having safe mode engaged and that keeps it from doing damage to the transmission,but it also means a problem has been detected ,a bad throttle position sensor will also cause this too happen or a shift solenoid,you,ll have to get it scanned though to find out whats causing that too happen,good luck.
