904??

Lockup and overdrive are not the same. A lockup simply locks up the torque converter down the hwy so it doesn't slip any. It only drops the rpm 100-200 rpm at the very most. Overdrive is a complete other gearset and will drop the rpm several hundred. They also made lockup and non lockup trans. all throught the yrs so it can go either way. You are correct that most truck trans. are 727's.