Lift up (jack) 1 front wheel & engage the tranny in Neutral & apply the Emergency brake, this will allow you to turn the prop shaft by turning the tire that is slightly lifted
and position the zirk in the best sport to get the grease gun on it. The zirk fittings do freeze up, I usually us something small to push the little ball bearing in the center of the zirk in and out a bit .... maybe a large paper clip end ? Remember to grease the slip joint zirk too.
For best results and U-joint longevity do this every 5,000 miles.