G&A,
The clunking that Jo-Anne refers to is nothing to be concerened about. The Sorento has a system where it drives the rear wheels under most normal driving and even on some off road terrain too. The higher spec models (XS/XT) have what is referred to ATT or Active Torque Transfer. This uses sensors to detect the speed of the rear wheels compared to the front wheels and if the difference exceeds a preset level the transmission automatically selects four wheel drive. The drive is engaged automatically through a clutch within the transfer box. If you put the car on full lock, whether going forwards or in reverse, and don't go very slowly the ATT system senses that there is a difference in speed between the wheels (caused by the differing radius of turn between outer and inner wheels front to back, and seeks to engage the transfer box clutch. Under these conditions car speed can vary slightly as the driver manoeuvres and the TB clutch keeps flicking between off and on, hence the clunking. If you do hear clunking either move more slowly or take it off hard steering lock, or both.The Sorento requires the rear and front tyres to be of equal diameter or the front a smaller diameter or else ATT can be 'confused'. This is not a restriction as in my experience (70k+) the front tyres wear first and so you either rotate tyres or as I prefer move the rears to the front and fit new tyres to the rear.
If you look at the two links you can find more info on ATT. Early Sorentos did suffer clutch wear in the transfer box and were fixed by Kia. Some owners had the TB repaired at specialist gearbox companies and costs were from memory about £750 with an hour each way to remove and replace. Around 05 Kia introduced a new TB oil which was more akin to an automatic transmission fluid and what was previously a rare event disappeared.
The TB oil is a specific Shell oil but difficult to obtain in small quantities but Kia dealers sell it as Kia TOD Oil Part Number 04500TA389. Also the auto gearbox uses a specific oil (not Dextron or ATF) it is Apoloil Red again from Kia dealers.
Here are some links. The technical one shows how the ATT/Torque on Demand system works (page 60 onwards). The ATT system is on new Sorentos so don't be afraid of it although changed to reflect their FWD primacy.
Good Sorentos go quickly, mine was reserved before it even left to Volvo dealer where I P/E it.
http://kiasorento.proboards.com/index.cgi?board=Technical
http://www.kiaownersclub.co.uk/forum/transfer-box-diff-oils_topic4474.html
http://sorento.kia-club.ru/Repair_manual/download/DOWN/BL%204WD.pdf
Good hunting.