Can't speak for the Ranger 800 or 400, but I ride with two different ATV clubs with Polaris RZRs in each club. People in both clubs have had problems with some kind of plastic cage in the front drive that holds the bearings or something, and several have had to weld on reinforcements to the 2" receiver hitch on the rear after bending them...
However, I've heard less complaints about Polaris warranty coverage than Yamaha's YES warranty program -- but I don't know how much the local dealer is influencing either one... Haven't heard much either positive or negative about Kawasaki or Arctic Cat warranty coverage -- I personally know only one Prowler owner.
Given the cost of extended warranties, I chose to buy the factory manuals and do my own service.
You do raise an interesting point, though. The Rhino and Teryx have true, locking front differentials, while the Polaris Ranger and RZR do not. (Don't know about the Prowler.) The flipside is that the Ranger also has an open, lockable rear diff -- i.e. it has a turf mode that will cause less damage to your lawn. The Teryx and Rhino have the equivalent of a locked rear diff...