I agree with with randal-s.

There must be a means of holding the load elevated, without relying on a motor. A brake on the cable drum (assuming it has one) would make the most sense.

If the load weight is causing a DC motor to run backwards, a AC motor is going to run back wards as well. Does the motor run backwards with the power off?

It would be less expensive to get a new lift truck with AC motors and controls than to try and update an existing truck. Guarenteed.

By the way, I am a proponent of the AC motors and controls.