We had a similar problem - a liberal application of WD-40 (per HWH) with the jack fully extended would keep it happy for a few weeks. Finally tired of the need to repeat the treatment and just had the jack replaced.
But first, if there are springs on the jacks, make sure both springs are still there. A replacement spring is a heck of lot cheaper than a new jack. Almost any decent RV service shop can order and install a new jack. We had one replaced in Texas a few years back and the tab was almost $800 dollars. The second one was replaced last spring by the shop that does most of our work, and the tab was under $600, including expedited shipping, so get a few estimates if you can. The replacement process is about a 40 minute project.
al