FRUSTRATED! need help with carrier swap

Put some thick grease on the end of the axle or play-doh(clay) on it if you can get it to stick. Put the axle in carefully as not to disturb the clay or grease. When it bottoms out remove it carefully & check the clay or grease on the end & see if it's mushed. That'll tell you if its the end of the axle interfering or if it's something else like a spline not aligned.
A similar method can be used to check the spline depth by greasing the axle splines with white lithium grease & intalling the axle as far as it will go then remove it & check the splines, the grease will be smeared where the axle went into the splines but not where it wasn't. Now, without disturbing the greased splines, slide it into the old diff & compare the depth. If it goes into the old diff further check new one for misalignment or thrust block interference. Don't cut unless you have to.

You ca also apply the parking brake good & tight to hold the opposite axle steady when you try to align it if it helps to have the car jacked up higher, since you will have the one drum off you must have the opposite drum on though obviously.