play in the rear wheels, what could it be?

[TTLex]

I was standing on my rear wheel the other day adjusting my GOBI roof rack and I noticed that my wheel was clunking back and forth as though it was loose. I hopped down and shook it forward and backward a few times and you can feel play in the wheel/ hub area. I did it to the other side and it has it as well. Its just the rears, the fronts dont do it.

I checked a buddy at the gym's hummer that stock and his does not do it.

What could this be, wheel bearings? I had to replace one of the front hubs last year, but it was squeeling which is how I knew it was bad. I would have never noticed this if I wasnt standing on the wheel. Also, what are the odds both rears would go at the same time.

Just to be clear, I have a 6" Fabtech lift with keys and spacers putting it at about 8" up and IM running 40's.