Vince is right, that would be a huge task.
There are other machines besides the 3200 that have the diff lock. 3240, 3225, and 3235. All the older versions of the 3200. 3240 is the Anniversary edition.
You might be able to buy a trannie and swap, but you also need all the linkage, pedal, brackets, etc, and cut a hole in your floor pan. Might be cheaper off going bigger, get it built in, more HP, and bigger tires.
If we were closer, I'd offer you my 3235 for $1800. It needs a few things, but has the diff lock, big tires and diff lock. I also have 3 decks to choose from, 48,54, and 60". I buy and sell Cubs as a hobby, but leaving next week for 6 weeks overseas. (work)
I also wonder what the loader will cost? I'm assuming around 3K? For the cost of a loader and your tractor, you could be real close to getting a sub-compact. I'm selling a Cub 5254 25 HP gas 4X4 with loader for ~6K. It also has the mounting kit for a mower.
I'm not trying to sell you anything here, just show the cost difference. If you shop around enough around your area, you'll come up with a great deal on a sub-compact, or 3000 series with the diff lock.
Good luck!