Hook up the trailer to your truck. With the weight off the jack, locate the hole in the side of the box on the down tube. While cranking the foot up or down, using a flashlight, you will see an inner hole, that you will line up with the outside hole.

When these are aligned, squirt a liberal amount of automotive oil through the holes into the jack's core. Run the jack up and down to each of its limits. Again align the holes and relubricate. Again run the jack to each of its limits. You should then find the jacking effort much less. If the effort does not change and the top gear box is lubricated, the jack has been damaged and may have to be replaced.