Could this just be the TPS? My 2nd gen has a high speed / low rpm shudder and then goes away when I floor it (also goes away if I press the OD button to downshift without any throttle changes).

Unplug your TPS (at the throttle body) and see if the shudder goes away. It will definitely run differently (slower) as it's some sort of limp mode, but I can not get mine to shudder without the TPS, so that's what I'm blaming... I'm new to Monteros, but this ain't my first rodeo.

Good luck!