noise from rear

[lesZ71]

I have a 2004 Z71 with the G80. Every once in a while I hear a short thump sound coming from what I believe to be the rear whenever going over uneven surfaces (like entering a driveway or going over a speedbump). The noise also occurs once in a while when turning corners at low speeds (less than 20 - 25mph). Could this noise just be the G80 and considered normal for these trucks? This is my first vehicle with a locking rear differential. I have a friend with a Detroit locker and his will make really loud noises whenever it kicks in...but mine is more like someone hitting something under the rear with a mallet.

[lesZ71]

I have discovered that the noise I am describing seems to go away whenever the rear of the truck gets wet. I noticed that when driving in the rain that I do not seem to hear the noise for a couple of days. So today I decided to test this theory a bit further, so I took the waterhose and sprayed the leaf springs and shackles liberally, then went for drive. I noticed no noises, even when turing into a driveway. I am wondering if I just need to spray some lube on the shackles and leaf springs....is there a recommended product to use for this...or will some silicone spray work just fine?