Or . . . they are rust-welded into the stake pockets on the bed. The tabs don't do anything unless the stakes (posts) are loose. Then the tabs keep them from popping up and out. Ignore the tabs until the stakes are wiggling freely all around.
Pressure is fine and it can work but if they're solidly rusted into the stake pockets, all you'll do is jack up the trailer by the rack wood. It's impact that will keep progressively cracking the rust and allowing the next application of penetrating oil to work in deeper. It's a repetitive process and sometimes it's pressure + impact. Bang up, bang down, bang the stakes, bang the stake pockets. Use wood or you'll dent something and that could make it mui-worse.
Although you'll find lots of threads on here for loosening things that "...work every time...", you'll often find out that one thing won't work for your situation. You just gotta keep trying them all -- PATIENTLY -- and eventually it works. I get the feeling you haven't read through that 116 thread that's offered above and if you didn't, you're missing a lot. Same thing, except that I doubt you wanna solve the problem with a Sawzall.
Good luck.
Steve