For a typical commuter XJ driven by an old lady (not off-roaded) ,
at while mileage do these components typically fail? (If you had to put a number guess on it)

1) U-joints
2) Hub bearings
3) Exhaust
4) Rotors.

 

When they start making noise that they shouldn't make or moving in ways they shouldn't move.

 

1) U-joint keep them greased and will last. Little old lady owned mine before me, i changed the original one out at 90k, after a dune trip
2) Hub bearings, same as above, you cant grease them.
3) Exhaust- untill it leaks or cloggs.Cant really put a number on it.
4) Rotors. 30k

 

Impossible to answer.

So many variables involved, you will not be able to realistically tell.

 

Granny can have a lead foot :P, but there is no real answer. Just wait for it to break, and replace it.

 

Also depends on what aftermarket parts you replace them with. I hate Vatozone u joints! They seem to have play after installation.