need help: ae86 diff - halfshaft problem

[cj]

I have the following problem:



I brought my car to a shop for raplacing rear shocks. When we lifted the car up, we noticed a problem right rear wheel doesn't spin.

My diff is 100% locked so i thought the halfshaft might be broken because of the excessive torgue when turning small radiuses (not drifting). I took out the halfshaft and notice that part of it has been worn out. What seems strange is that the worn out part is at the very end i covers only a fraction of the halfshaft (as seen on the pics). Now i'm planning to take apart the diff and see if anything's wrong.

I do have a spare halfshaft, but i'm not sure if that will solve my problem. Does anyone know how deep should the shaft go into the diff? Any further advice?