The passenger side rear driveshaft looks to have slightly seperated itself from the diff and a partial oil leak has followed.

Its still attached, but when comparing to the drivers side driveshaft, theres probably about a 7-8mm gap between driveshaft and diff. In the 7-8mm gaop you can see a bright silver part of the driveshaft still attached.

Any reason this has happened? Should it just re-attach or is the driveshaft knacked?

... cars a 1998 STI

 

As long as you have not fitted any other trailing arm sets or w/e this shouldn't happen.

You can force the shaft back into the diff. It just clicks in the diff and should stay there....

 

there`s a kind of circlip that fits into the groove on the shaft that holds it in place