The usual way to prevent slipping is to use a worm reduction gear. But I don't recall seeing one with an encoder.
Why do you need the encoder? A stepper motor will be easier to program than an encoder.
That stepper motor claims to have a holding torque of 2kg-cm. What torque was your 180g load imposing on it to make it slip? How were you driving it?
Have a look at stepper motor basics.
You haven't told us the diameter of your winch drum. If you have huge drum, then the motor cannot lift much. A small drum will work much better.