I don't expect a definitive answer for this.

My truck has a random clunk on occasion. It started maybe 2000 miles ago. I cannot determine whether it's coming from the front or rear. I can't feel it through the floorboard. With all the windows down, and cannot hear it. Front end suspension is tight. Shocks are good. There is no play in any of the driveshafts/drivelines. I hear it when I'm around town. I cannot reproduce the sound, it just happens.

Any ideas?

Mark the drive shaft, pull the drive shaft and grease the spline where it goes into the trans. If you search this site you will find photos and instructions. I had the same problem last month. Search along time and finally grease this area. Problem was solved.
Chuck

Thanks, I'll do that. I did have the driveshaft off a few months ago for U joints, maybe I wiped some grease out or something.

I had a thump in my floorboard for the longest time and just changed out the tie rod end links and its gone... Changed because the rubber was shot not because i thought it had anything to do with the clunk....

I had this clunk and did the driveshaft greasing with no improvement. I discovered the problem to be loose cab bolts. There are 6 bolts that hold the cab on to the frame, and I had previously replaced the rear rubber bushings to help with the rough ride. Turns out I didn't crank on them enough when retightening.