trailer jack shaft wont raise

[woodskiff]

I'm looking at the 5k bolt on tongue jacks myself. I've heard that the top crank posts can be a nuisance if you mount a box on the front. Front mount crank with the optional foot from etrailer. 52 bucks.

 

[JJZ 109]

Up here in cold country I've seen water (snow and ice) get inside jacks like that and freeze up busting the nut the worm gear rides on but doubt that would be your problem.
As mentioned, take it apart first to see if it is a simple fix. Here's a 5K bolt on replacement for about $26.
Others are available at various prices.

Pro Series Round, A-Frame Jack - Topwind - 15" Lift - 5,000 lbs Pro Series Trailer Jack PS1400600303

I also had one gum up on me on a 6x12 Cam dump trailer. It would only turn about 1 1/2 cranks in either direction. I was just going to replace it, but I friend took a look at it. Said that it had some old, dirty residue inside. He cleaned and lubed up the thrust bearing and shaft, drilled and put a grease fitting in it. The thing works better now than when it was new.