You won't hurt anything if it all comes out. There is a spring and piston in there that may fall out so don't lose them and make sure to put them back in as they came out. Sometimes it's just easier to pull it all out and seperate on the workbench!
Let me take a guess here, you sit at a desk all day but you've changed your own oil once or twice and own a shitload of shiny tools so you think you're an auto tech?
I'm not arguing with a pompous ass on an online forum. Good luck OP!
Different manufacturers vary in size greatly. Even if all four tires are the same brand/size but say the rears are worn and the fronts are brand new will be enough of a difference to upset the 4x4 system.
Different brand tires will be different sizes. This is your problem. All four tires must match in any 4wd/awd vehicle. The 4x4 system is binding due to the different size tires.
It's also possible that the front and rear differential gears do not match.
Can't really say what you need part wise without a proper diagnosis but shouldn't be too expensive. The vent valve is probably around $50. That's a guess!
The fuel tank is not venting. Could be the tank vent, vent valve, evap canister....
Any codes? Probably need to drop the tank and check valves and hoses.