I had the same problem and here's what you do:
You must be very careful to NOT pound on the end of the axle or apply any heavy stress while the differential is still "in play". Pounding on the end of the axle will flare it and you will essentially have a riveted wheel assembly that will never come apart. No amount of penetrating oil or wheel pullers or any such will get one of these beasts apart, and you do NOT want to damage the differential gears, so do the job right, which is:
Remove the drive chain from the small sprocket.
Unbolt the two flange bearings--one on each side of the frame--that carry the axles. The frame is slotted for the axle.
Jack the mower up about 6"
The axle, differential and both wheels should drop down thru t he slots and clear of the tractor.
Remove the nuts that secure the brake drum and big sprocket to the end of the differential.Separate the differential.
Remove the spring clips that hold the gear on the spline on the end of the axle within the differential half
Don't be too worried about how much you've disassembled the mower--nothing you have done so far is risky, difficult or takes much skill and the only alignment or adjustment you must take care of is the axial alignment of that big sprocket with the little sprocket on the tranny. Pay attention to where all the spacers and washers are on t he axles as you proceed and put everything back the same way. There should be very little endplay in the axle, otherwise the drive chain will keep dropping off.
You now have two half-shafts with wheels stuck on the ends.
Get a BIG arbor press---we used a 10 tonner and it was heavily loaded!!
Apply lots of heat and then press the end of the axle out of the wheel hub.
Clean the hub and the axle thoroughly and do NOT forget to apply lots of anti-sieze compound on reassembly. Once you've gone thru all this grief you will probably see the wisdom of a yearly wheel removal/replacement.
