On a side note for some of the cars running older unserviced Jag/IRS rears... , any time you suspect something might be wrong with the U-joints, don't wait and see what happens.
Over the last few years especially I've come across a few close calls with friends cars where we talked them into going ahead and pulling out the rear or the 1/2 shafts at least to check things out more closely. For example, finding one U-Joint cracked but still together somehow, and and the other U-joint just starting to crack, with a mildly twisted stub-axles, etc... When these things let go, its not a great experience since the 1/2 shaft IS the upper control arm. When the U-joint goes, so does your upper control arm, and maybe the entire back corner of your car if you are one of the unlucky ones. Seriously, check it out or have it checked out by someone professionally if you have the slightest concerns about your old Jag/IRS.
One clue is to raise the car and check for play fore/aft on the axles. Should be snug with minium play, if any. If you start hearing clunking when you are just taking off or transitioning from reverse to first, check it out. .... Good Luck.
