It's usually pretty hard for the splines to get stuck because they are normally splashed/bathed in gear oil. If I remember correctly you had some differential damage, so you might have some metal shavings in there jamming the splines.
If you don't care about the axle shafts, you might loosen the splines with some hammer blows on the end of the axle shaft. This might force the axle in a little bit and result in it being loosened.
If you are trying to reuse the axle shafts, maybe you can come up with a "pusher" to push out the shaft from the inside of the differential case. I'm thinking of something like a threaded rod cut to just the right length, and then a threaded coupler that has been threaded onto it. You hold the bolt in position and then turn out the coupler. This will give you a lot more force than you could create with your hands. I'd put the turning coupler against the axle shaft, because if you scratch that end it doesn't matter much.
Since you have the whole assembly down, if it were mine I'd go ahead and replace all the oil seals too - the pinion seal on the front and the two axle seals.