you CAN buy the ird parts as required off ebay (barings/seals etc) and the likes, personaly... that wasnt the way to go as the ird was getting noisy anyway. Has the car got noisier recently? mine was particularly bad when driving slowly.... The main expense with the ird is the crown wheel + pinion, if they've gone, its just as easy + cheap to get a recon unit rather than try fixing it!!
Im not saying its deffinately the same problem... be worth getting it checked over sooner rather than later..... Dont forget to check the barings either side of the viscous unit aswell, they must have been in terrible condition thenb blew apart on mine, which finished the ird off!