I don't know about the rear wipers, but my front wipers were terrible until I replaced the linkage, (used for about $40). Now they work like new. The linkage was slightly bent and the wipers did not hold to the windshield very tightly. Anyways, one possibility.
For the rear wiper my money is on a broken wire in the upper tailgate hinge area. I had a bunch of broken wires in there.
Carefully pull back the headliner at the tailgate area and you should be able to see the wires. Not much room to work though. Don't try splicing the ends together (it'll break again very quickly), simply add a length of STRANDED wire to go through the hinge area and splice that on each end to the original wires leaving the new wire running through the hinge area. That is also a good spot for the rear defrost to break, and for the wire that allows the rear wiper to return to it's resting spot to break.