My first guess is it is time to clean and grease the Jack Screw. It sounds like the top part of it is gunked up. I don't know if your year/model came with a lube point or not. If it didn't, I have heard of members pouring a little 90 weight gear lube down the top of the leg in an attempt to lubricate the Jack Screw without having to remove and dissemble it. Not too much gear lube as it can make a mess when it leaks out the bottom.
Here are the instructions for removing a leg provided by Venture Manufacturing. They supplied landing legs for many years. It should be similar for whatever brand landing leg you have.
Landing Gear Removal and Installation.pdf
Then if you want to disassemble the leg, use these directions.
landing gear rebuild instructions.pdf