Take the top cover off the jack itself, should be one or two little sheet metal screws holding the cover on. Pour a little engine motor oil on the screw/worm gear, put on your best cranking arm and twist away. The grease gets dry and stiff, the engine oil will loosen the grease and free up the gears.... That is if there's no stripped or gaulded parts. After it's free, use a yard stick, old piece of molding, a clean stick, etc. to smear some grease back onto the screw.... Spend a little extra coin on the lubricants, purchase synthetic oils and greases and you may never have to lube this jack screw again....

Hello Cobbler ... Welcome to this forum.

The trailer jack has two sets of moving parts.

The first is a pair of gears at the top.  They convert the horizontal rotary motion of your crank to a verticle rotation of a screw shaft.

The second is a nut on the vertical screw.

Lubricating the gears is easy. Lift the top cover of the jack and grease the gears. There may be a bolt securing the cover.

Lubrication the screw is a bit harder.  hitch the trailer on the truck.  (this will support the trailer) Look for a hole in the outer tube of the jack.  The inner tube will be blocking it.  Starting with the jack fully retracted, extend the jack leg while looking at the hole.  Eventually a hole in the inner jack tube will line up.  The foot tube inside the jack MAY obstruct the second hole.  You'll have to lower the foot too.  Squirt some heavy oil on the screw and extend and retract the jack a few times.

It should be easy again.