Chances are that your rear wiper is suffering from the common problem of a corroded and binding output shaft. How easy this is to fix will depend on the year, but I'm not sure exactly where changes occur. In older ones, such as my 87, you could take the motor out, and open it up, and press the shaft out once the motor is open, clean it up, relube, and you're back in business.
In later ones, such as my 99, the motor is held together by rivets, and you can't open it up unless you're ready to do some rather major reworking. I did so on mine, but it helped to have a milling machine to mill the rivet heads off, and to drill out the holes for bolts. You could probably cut them off with a Dremel tool, or even chisel them off, but be prepared for a bit of a fight.
If you're lucky and it hasn't gone too far you might get a little extra life by just taking the wiper arm off and trying to get some lube down the shaft, but it's a long shaft and gets dry far down. It might help if you take the arm off and lube it while it's running. Then again it might not. If you wait until it stalls, you'll burn out the motor or strip the gears, so if it's slowing down and stopping in the wrong place, consider disconnecting it until you have time to fix it.
As for the axle shaft, if it's just clinking and clanking a bit, you probably have some time. I've gone pretty far on rattling joints. But if it gets enough play for the two yokes to get out of line and interfere, it runs the risk of binding up on a curve, tearing apart and trashing everything in its path. The yokes on the shaft are pretty strong, but I don't think you want to find out whether they can win a fight with a jammed joint at highway speed. I seem to recall seeing some nasty pictures of trail carnage involving busted joints taking out a knuckle. You might want to avoid that.
