I have a slight clunk when i let out the clutch...i think it may be diff bushing or u joints.. Can the u joints be replaced or do i have replace the half shaft 78 280z? Please help..

Have you checked the infamous front differential mount? Those are the most common source of a clunk when shifting. Normally a bad universal joint will cause a vibration. (At least they always have in my experience.)

Someone else needs to correct me please, but I think that after some point, which I thought was the 280Z, the universal joints became non-replaceable. Since 78 was the last year for the first generation Z, I assume that the original U-joints were not replaceable. That should in no way be assumed to mean that YOUR U-joints are still of the non-serviceable variety, since the car is 30 years old, and who knows what has been done to it in the interim.

You can check the diff mount by putting a jack under the diff pumpkin and applying lift . If the pumpkin rises first then the car starts of rise. the mount is bad. All should rise at the same time or vary nearly so. Walter is right about the u-joints. There is solution for these , but I haven't done it my self so I cannot advise. I am sure it has been covered on this forum in the past so see if you can fine the information with a search. Or someone may come here that knows.

On my Z the u-joints are replaceable. Gary