How do i change the u joint on a 2001 durango?

when Driving i hear a clanking sound,and when i put it in 4wd it makes a grinding sound i have had several people including a mechanic and a durango owner tell me it was the u joint... is this a difficult job to do...i am no mechanic but by no means helpless just looking for some advice from somebody who has experience......any advice would help.......Thank you, Phil