NewB 760 U Joints

Ok, I have an 89 XJ w/ d30 and d35 w/ 3 1/2" of lift. Right now I have 330,000 miles and my car and the U-Joints have never been replaced so I think its in need of some changing. While in 4 wd sometimes i'll hear a clunk noise up front and i think it has something to do w/ the U-Joints and front driveshaft. Now I know you can get these joints from the 95 and up XJ's, but can you also switch out the front driveshafts w/ it? I also wanted to know how hard it is to replace the U-Joints and what else I would need w/ the U-Joints to replace them like bushings. How expensive would that be and do you have to go to a junkyard to get it or can you just buy it at an auto store.