Ordinary AC motors & position control

I'm asking for some basic knowledge. I've seen some motors that
normally spin free lock up pretty solid on a position by use of
a "dither" frequency to equalize both directions of rotation. Slight
adjustments can move the motor one way or the other. Now can this
principle be applied to ordinary AC motors? I understand it works with
DC motors.

If not, is there a way to easily convert an AC motor to DC?

The reason I ask is new AC motors can be inexpensive relative to servo
motors of similar power. In my application I'm more tolerant of
backlash than expense.

Thanks in advance.