If it only makes the noise and vibration under load, I'd suspect an internal issue. Before pulling it and tearing into it though, you may want to try this:
On the front driver side (at the end of the worm shaft) is a cover for a band drag brake that is supposed to hold the load and keep the cable from unspooling when the PTO and clutch are not engaged. Take that cover off and see if the band and drum are crudded up. Most are full of rust and dried crud. The band can sometimes allow the winch to free-wheel in one direction and bind in the other.
Loosen it with the nut on the bottom of the housing, free the band from the drum (if it's visibly binding) and flush the crud out Leave it loose and see if the noise is still there.
If it's gone, clean the gasket surfaces, put on a new gasket or use Permatex and replace the cover. Then adjust the band brake as per the -10 manual.
If the brake drum is rusty, set the brake band a little loose until it gets work polished and be very cautious using the winch on a slope until you are sure the drum is clean and the brake is properly adjusted. Running the cable in and out under power a few times should clean it up fine.
If the noise is still there (with the brake loose) you've got internal issues to check up on.
At the risk of being insulting, have you checked the lube status of both gear housings? Are you sure the engaging lever on the winch is fully engaged?
Good luck.
Lance