I had the same problem with one side going in... but I did not raise my wagon enough to drop the 3rd member, and I was on the ground...not even concrete ... so I figured the distance with the new axle was just too much... I used a brass rod and had to knock it in .... me holding it and helper hitting on end of rod behind the car... it was SO close... could not believe it could be that close and not go on in... it may just be offset pressure on yours.. that is a tight fit...
Or .. is there a spline that is different from the others ? an alignment spline ?