In my limited experience with Toyota half shafts is that replacement axles seem to come with a slightly larger c clip than the one on the old axle. I have always had to swap the c clip from the old axle to the new one to get it to seat. With the right c clip, the axles slip right into place.
Might have to go to a dealer and see if they can order a new c clip for you. Maybe you could get one from a junk yard if they have any old axles laying around with the c clip still on them.
Edit: Your mechanic might try this, first remove the c clip and slide the axle in place just to see if it will go all the way in. It wont click into place but if it goes in all the way, that is a good sign. If that works, then take the c clip and grind down the ends like they would if they were gapping new rings in an engine. This will allow the c clip to reduce down to a smaller diameter. Take 0.05 to 0.10" off at a time until the axle lockes in place like it should.