I had the same thing, just popped up after a long trip with weight in the box. Went to my dealership and they had it for about 1/2 hour and fixed it. What they said was the plates in between the rear springs were dry they greased them up and fixed the problem. Note: that was 8 months ago and no rattle/squeak yet.
The only work I've had done in my truck was the suspension tsb like 2 months ago I believe. For the last couple of weeks I've been hearing an annoying rattle/squeak from the area under the rear passenger seat. I just assumed it was all the crap I had under the seat/behind the seat. Last night I took everything out of the compartments and it seemed to if anything get louder, I hear it as the truck shakes or I hit the brakes. I looked underneath and didnt see anything blatently loose under there. Has this happened to anybody? Do you think the dealer left something loose down there, when they did the suspension tsb?