Something odd when cold

[SykoStroker06]

Getting what sounds to be like a sticking or slight clunking noise coming from the rear end when temps outside drop below 20 degrees. Just started doing it this winter. Have had rear lube changed in August of 2012. Rear pinion seal changed at same time. Universals are good. Center bearing looks fine. It will only do it once a day when truck is sitting for more than 5 hours in very cold temps. If I open the drivers side door when moving forward or reverse can hear the noise. At first just thought it was from snow/ice dripping off and freezing to the ground, but made sure it was dry in garage the last time it happened. Today temperatures were above 32 degrees and did not make any abnormal noises. Also had axle shafts greased last time the grease seal was replaced. Truck is 2006 F-250 4x4 CC shortbox with 107xxx miles on it.