It sound as if something is amiss, causing resistance. You should not be having a problem.
Are you possibly placing too much downward pressure on your mower deck, driving the wheels into the soil?
Do your mower wheels rotate freely when you have it picked up? Sometimes they pick up grass and bind.
On a "C", a person is in 2-wheel drive when in 4-wheel steer or 2-wheel steer under normal operation. To get 4-wheel drive, it is necessary to first put the TC in 2-wheel steer, and then depress and hold down a button that engages 4 wheel drive.
If you are stuck with a mower, the best thing to do is detach the mower, try to get out without the additional weight. Then, once out, hook a chain from the lift arm to the mower, lift, and drag the mower to better ground to re-attach it, minding not to cut or tear out the hydraulic hoses in process.
Having a heavy ATV winch which you can plug into your rear receiver is great for getting un-stuck, and having a receiver you can put on the lift arm so the winch can also go in front is also great. With enough cable you can double the pull by using a pully which might come on the ATV winch.
While ballast is helpful when pushing down on a solid surface, it has the opposite effect when pushing on a soft surface. If you are stuck, the heavier you are, the faster you sink.