Shudder in truck

I have a 2012 single cab 4x4 auto. I bought it slightly used low miles. I have full warranty on it still. My problem I'm having is that sometimes when I'm slowing down the truck will shudder ever so slightly for a second or two. It only happens once a driving session on the first time I slow down and usually only after it has sat for a while. I mostly only feel it in my feet as in it doesn't rattle the truck or anything violent. It will start and stop on its own with no cause or change from me. I have even had it happen where I applied the brakes and it began and I took my foot off and the shudder continued momentarily during free roll. I have made several comments at the dealership about it but they were stumped but marked down. It seems to be around 1st or 2nd gear. The dealership said I would have to leave my truck for several days for them to diagnose but I would like to have some info first so any advice is helpful. Also seems like it occurs more in the colder months. Thanks.

 

Any thoughts or ideas of what to check would be greatly appriciated I don't want to get the run around from the dealership.