Shudder when coasting at highway speeds

[Burnout]

Noticed this problem for the first time today taking a small day trip. When I'm coasting (either not using the accelerator or using the "coast" deceleration button for my cruise control), as the speed goes from a little under 60 to around 50, the truck begins big shudders. The engine has a "winding down" sound, but that might be normal and I just haven't noticed it, but then the truck shakes as if I just went over one of those "Wake your ass up" bumps in between lanes. The sound then starts again until it makes the bump again.

This doesn't happen if I have cruise control on, or if I'm using the accelerator at all. And doesn't happen when I'm faster or slower than that ~10mph range.

Any ideas?

 

[Robsbrd99]

Start down the highway at 60 or so, then, with your foot on the gas, lightly depress the brake. The engine should idle up slightly and then go down after you get off the brake. Im guessing the torque converter is bad.

 

[Burnout]

Start down the highway at 60 or so, then, with your foot on the gas, lightly depress the brake. The engine should idle up slightly and then go down after you get off the brake. Im guessing the torque converter is bad.

Well crap. :/

So by doing that - lightly braking and seeing the engine idle up and then go down after I let go - does that show that it might be a problem or that it isn't the problem?