I have a 2000 and as the rest of the pack, they eventually got sick. IN fact, they were already sick when I had bought the car in 2003 from the KIA dealer...and unknown to me at time of buy, the spring position had been reversed in the hub so they were in fact engaged full time. I quickly realised the shafts were turning all the time, diassembled the hubs, seen the problem, put the spring back in normal position and got good service during the summer. (In fact, the vacuum solenoid valve was dirty and sticky and would let vacuum seep through even when not asked for..so I had to correct that problem to get proper operation..and several have had this problem with the valve.)
QUestioning the dealer, they told me they had done the KIA recommended fix and the hubs would be changed free of charge when I want (car was still under original KIA warranty plus I had the standard used car warranty).. WHen winter came, they could definitely not be used and created all kinds of havoc...and Warn auto-mechanical with the KIA label were installed at no cost to me.
I have good service with the new ones but they have limitations in the way they must be engaged/dosengaged that the vacuum did not have..anyway, they work perfectly.
IN case it is of interest, installation instructions:
www.geocities.com/lmp4203/kiasport/hubinstall.pdf