when you slide the slip yoke of your tailshaft into the rear of your gearbox, not only does it match up the splines and slide over
the ouput shaft, it also slides into a bushing thats there to keep it centred and running true.

the bush will look something like this..




benny was spot on. These bushings should last decades, so fixing the cause at the same time is definately the way to go.
To replace the extension housing bush, id expect you might have to split the extension housing from the gearbox and press the old bush out, and a new one in. i could be wrong on that one though

good luck