See how much movement you have on the sprocket with the rear brake locked.
If it's more than just a bit change it. In fact if it's the original it might be a good idea to change it anyway. I went with the T-Bird Sport cush drive about 30k ago and it's still tight.
It does take a slight bit of persuading with a soft mallet (I used a four pound dead blow) to get it in but well worth it.
