Universal Joints

ok i went to change out my rear joints and ran into some questions. i have a 91 manual 4wd EB with 3.55 gears i went and got the joints for it and came home to instal them and after i got the joint in i noticed (very plainly) that it was way to short. so i removed it and went back the auto store pissed cause im 30 minutes away from the nearest one (auto zone). well the numbers matched they gave me the right ones so the comp. says . we check it out in the paper manuals and yep i got the right one so it says but it damn sure didnt fit , ended up using aa full size ford bronco joints?? that had me stumped seeming they are a bit bigger but it works and i got all the snap rings in. im just curious as to why i had to use those and did any one else? any tips???