2001 gmc sierra 160k

I have had this problem with my truck since I have owned it. Going from stops to starts I hear a sound in the rear end it sound kinda like a sound a seal would make. I have replaced all my u joints to greaseable ones... the sound has never gone away. I have had this truck for about a year and it has done it since week one... it seems like it happens at every stop start now... should I replace U joints again? Any suggestions would be helpful

If the sound is a clunk when you start and stop it could be a known problem with the slip yoke, there are muiltiple threads in this forum and the silverado forum.