The PVH system is prone to failure, but nothing that can't be fixed and not a reason to avoid the truck. Before agreeing to a price I would verify that the system works and the hubs actually lock in when engaged. Would also be a good idea to check that the transfer case shifts and engages, as the shift motors can fail if not exercised regularly. If either one does not work, it isn't the end of the world, but they may be good points for price negotiation.
EDIT: Also want to add that unless the shift motor happens to fail in 4wd, neither of the above situations will affect normal 2wd driving. 4wd would just be non-functional until they were fixed. From my understanding the shift motor practically never fails outside of 2wd, because they usually fail from lack of use. If it does fail the motor can be removed and the t-case shifted back to 2wd with a pair of water pump pliers (aka channellocks).
Generally when the PVH system fails it is due to a vacuum leak in or around the hubs. Sometimes it's the hose from the frame rail to the knuckle, some times it is an o-ring or seal in the knuckls. You can usually disassemble, locate the leak, and repair. There are also workarounds to manually engage the PVH hubs if desired/necessary. Manual lock hubs are available, I'm not a big fan of what is available for this application, but they are the only alternative to the PVH parts for keeping locking hubs.
IIRC when the shift motor failure is usually due to the control board in the motor that reports position. Some people have successfullty disassembled and fixed them, but the motor is easy to replace since it just mounts to the back of the transfer case with three bolts.
If you wish to ditch the locking hubs all together it can be converted to live axles where the front hubs are full time engagd by installing the components from a 2001+ Ranger. My 99 got the manual hubs 20 years and I recently did the conversion to live axles. Allegedly some fuel mileage can be lost with this conversion, but I'm not sure if anyone has ever documented solid proof of that. Typically when I've seen reports of lost fuel mileage, some other change accompanied the conversion that could also account for the lost mileage and my own conversion is no different. Usually a lift, larger/heavier tires, change in driving habit, or in my case a complete drivetrain change with a mildly built AWD V8 swap (and likely change in driving as well). In addition to removing a failure point, IMO one plus to doing the conversion is that it opens up the available wheel choices since you don't have the large locking hubs to worry about clearing.
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