There are so many things that it could be. Even though you didn't have a problem before lifting can create new stresses on parts and over time can wear to the point of causing problems.
For those who are more involved in lifts, I recommend a full list of exact parts used (from lift kit to drive shaft to other upgrades you might have made), types of tires and rims, how long it has been lifted, how you drive (all road, some off road, major mudding, etc.), and anything else that is unique to your truck. Has problem been noticable from near time of lift or how much later, did it just appear one day or gradually became noticeable? Too much information here is never a problem. Good luck
