I'm not going to state that I have never heard rear leaf springs creak but to say that it is a "normal occurance", especially on a newer truck would be a stretch for me. There are certainly things that I can percieve as factors that may contribute to leaf spring creak such as usage and enviornment. Different enviornments such as sand, salts, mag chloride (used for snow removal) etc.,can potientially lead to contamination and abrasion. If you are an individual that uses the Raptor to its potiential and the leaves are being seperated with large extensions, then this may lead to contamination as well.
You might consider these things BIRD and then if you do not feel that they are a factor, politely suggest to dealer that you would like to have the insulators replaced. If they are abraided or contaminated, the dealer may not warranty them. Each dealer is different in how they handle a situation like this.
FSM
The WD-40 suggested by A.I.I. may be a decent idea as this will not attract as much debris but rather help keep it away and retain its lubrication quality. If all else fails, it's worth a try.