Bring the old one with you to stare and compare with the new one, the difference is in the size of the cap.

There are some aftermarket joints that have a grease fitting in each cap, these work well! Spicer alsio makes a greaseable joint, just add a -1 to the part number.

This place has good prices:
http://completeoffroad.com/c-64310-u-joints-d30-u-joints.html