Clunking noise at 45 mph

[materbri]

I have a '99 explorer. Took it in for inspection and had 2 used back tires put on. When I left the garage, started up over a hill, and when I hit approx. 45 miles per hour.....there was a horrible clunking noise....on a rotation...sounded like a flat tire. After test driving it, it only happens on an upgrade at 45 mph and stops when you leave your foot off the gas. I'm thinking it has something to do with the tranny, but hoping not! It does it in all ranges of 4 wheel drive. Just wondering if you have any suggestions of what it might be. It's strange that the noise was not there before the new (used tires) were put on.

 

[TPLYNCH]

If the difference in tire circumference between the front and rears is great enough, the Auto 4x4 may read the different tire speeds as slippage and be engaging the 4x4. How old are the front tires? Make sure the tire sizes are all the same, that they are all at the same tire pressure as well.