Thanks for all the replys.
"Bad gas maybe?"
It has done it through many tanks of gas from different stations.
"When you say "gears down" Does this mean you are slowing down and the truck downshifts? Is it at this time it makes this odd noise?"
When it slows down to a speed that puts it at around 1500 RPM, and from a stop up to about 1500 RPM you can feel the roughness or grind (though no metallic sound). You can sense a little vibration in the steering wheel. It almost feels like the trans is working harder at and below 1500 RPM with the A/C on but do not feel it hard shifting. Also, it seems to not be as bad even with the A/C on until it is driven for a few miles. I've had Rangers with 3.0's for about 15 years and never felt this. That is why I was wondering if it was normal for the '08' 3.0 or transmission, did they change anything from previous years engines?