Every book of mine says you cant replace the u-joints on a 1975 280z. Rockauto sells them. Whats the deal?

Which ones?  There are six.

Here is the info from the Nissan Factory Service Manual book.  There are shops that know how to make the propeller shaft joints replaceable but it's not standard practice.  somebody just posted about it.

 

Its the propeller shaft and your post shows its not repairable. Wonder what they did yo make it odd. Rockauto sells the ujoints but i doubt they fit.

Here is that recent thread.

 

Ok, removing the driveshaft is not as easy as it sounds. Whats the trick.

Remove the bolts at the back of the driveshaft.

Getting at the bolts is the issue

Had the same issue on the 78 280. Some one suggested loosening or removing the rear sway bar. Once that was out of the way I had access. Another trick is once you have the box end wrench on the nut spin the shaft by turning the rear tire. This will wedge the wrench against the frame and break the nut loose.

17 minutes ago, ennisdavis said:

Getting at the bolts is the issue

I didn't say it would be easy. 😉

The thing is that you don't want them to come loose easily. I had that problem when I bought my 260Z...and came back again after I got it fixed once. It's been over 5 years since I messed with those bolts. A stubby wrench may work in there better. Of course, you lose some leverage.

15 hours ago, ennisdavis said:

Ok, removing the driveshaft is not as easy as it sounds. Whats the trick.

Do you mean the propeller shaft?  The driveshafts are the two to the wheels.  It's a Nissan thing.

Find a wrench that fits the nuts tightly and will wedge against the body if you turn the shaft.  Then turn the shaft to break the nuts loose.  You should have the D head bolts so all you have to worry about is the nuts.  Find wrench, fit wrench in right place on nut, stick screwdriver in u-joint to turn shaft, wedging wrench against body.  X 4.

Once you get the nuts off you might have to tap/beat on the flange to break it loose.  There's a raised centering circle that fits a recess in the diff.  It's a tight fit.