It usually gives no problems, but when it gets wear in the joints, you can soon tell; you get some rear-wheel steering, which makes the car feel like it's following tramlines on corners, makes it wander in a straight line, and much more susceptible to cross-winds. 

There are several bearings that can go.  This article covers the wishbone outer fulcrum.  It's a weak bit of design, because the fulcrum is mounted in tapered roller bearings, but they only rotate about 5 degrees at most, and then it's oscillation not rotation, so all the wear is concentrated.  Add the primitive oil seals, and it's amazing they don't fail more often. 

The workshop manual is very confusing when it comes to overhauling the fulcrum shaft bearings in the rear hub, and at least in the XJ6 Series III version seems to give incorrect instructions. (And before we go further, be clear: we're talking about the fulcrum shaft bearings, not the drive shaft bearings.  The drive shaft also runs in a pair of tapered roller bearings in the hub, but they're much bigger (about 2" diameter), and they need 0.001" to 0.003" clearance) 

The important point about the wishbone fulcrum is that the two tapered roller bearings in the hub should be preloaded by between 0.001" and 0.003".  The two bearings are separated by a spacer tube and some shims over the fulcrum shaft.  You insert extra shims to get play, and assemble the bearings and the grease seal tracks, then measure the play.  Now  remove shims of 0.002" more than the measured play to give the preload.  For example, let's say the play was 0.014"; we'll need to remove 0.015"-0.017" to get the preload (the shims come in different thicknesses, so it should be possible to arrange a thickness in the correct range). 

That gets the correct spacing along the fulcrum shaft, but now we need to get the correct clearance between the bearings and the wishbone itself.  So there are more shims here.  The workshop manual tells you just to reuse the old shims, but that seems strange.  Given that we've had to adjust the shims internally, we'd expect to make a corresponding external adjustment. 

The manual's first problem is that the pictures do not show the shims clearly.  All the shims have a 0.5" inside diameter hole to fit over the fulcrum shaft, but the ones between the bearings and the spacer tube are about the same diameter as the spacer (something like 0.625"), whereas the ones between the bearings and the wishbone are much bigger diameter (about 1").  If you use the large ones between the bearings, you'll block the grease flow to one bearing or the other, causing it to fail early.  My overhaul kit came only with large shims, so I trimmed them to size with a pair of tin snips. 

The second problem is that the manual tells you to insert the extra shims, then apply full torque to tighten the bolts before measuring the play.  Full torque (over 100 ft lb) is more than enough to crush or dish the shims!  I tightened it until I could feel the nuts binding, no more - probably 20 ft lb. 

We still need to measure the clearance of the hub carrier in the wishbone itself; that's where the big diameter shims get used.  I found it relatively easy to measure the overall width of the fulcrum shaft pivot assembly with a 7" vernier calliper.  You could do this at the same time as measuring the play to calculate the preload.  Measure the width of the entire fulcrum shaft bearing assembly from the outside of the grease seal tracks; then subtract from that the thickness of the shims.  In our example, if the width across the tracks was 6.253", and we removed 0.015", the result would be 6.238".  Now measure the width across the inner faces of the wishbone where the fulcrum shaft goes across; subtract the previous result to get the thickness of shims required.  In our example, if the width was 6.250", we'd want 0.012" of large diameter shim. 

A last consideration: the manual says to put the shims outside the bearing assembly; that's not easy, as we're creating 0.002" preload, so there's a little pressure on them, which makes them slide out of position as we insert the hub carrier.  I put my shims between the grease seal track and the bearing itself; the track is too wide to slip out of place. 

The grease seals are just felt rings; you need to soak them in oil or grease, or they act as wicks to carry water into the bearing - not a good idea!  And the manual doesn't show the assembly of the seal carrier too well.  As far as I can tell, the assembly should look like the following: