8 3/4 Axle Issues

My guess is the inner and out axle gear splines are not lined up on the left side, Bolt the right side in and then your going to have to turn the left axle a little until the splines line up. It won't turn easy or shouldn't, but use a pry bar on the studs or bolt a wheel to the axle and turn it and tap on the end of the axle.