Strange feeling in 2010 Tundra DC

So I just started noticing a what I can only describe as a "taking up slack feeling" when starting from a stop. I'll try and explain it as best I can. Don't laugh. It feels as if there was a giant carpeted cube in the bed that is 1/4 inch shorter than the bed itself, and when I take off from a stop, it slides back the 1/4 inch and bumps the rear tailgate. And then when I come to a stop, it slides forward toward to the cab. Its like a soft thud. The bed is completely empty and I don't remember feeling this before. What could this be? Any ideas?

 

I sometimes feel my transmission stumble between 1st and 2nd gear. Like when I'm driving in a parking lot. I'm not concerned about this because all other times it works as it should.

 

This doesn't feel like a transmission issue to me. It's like something sliding back and forth when I stop and go.

 

Sounds like slop in the drive-train. I'd look at the U-joints first. Make sure they are greased up.