Filling the rears with CaCl and adding rear wheel weights will be the single best investment you can make for a 2WD tractor, or nearly any tractor for that matter, 2WD or 4WD.
You can have all the power in the world and brand new tires, but it won't matter at all if you don't have enough weight to transfer the power to the ground.
Note that the above is entirely dependant on what you're trying to use the tractor for. If you're just pulling a mower, you probably don't want as much weight, or you'll dig into the grass. You need enough weight for the job you're doing.