ok, this is gonna be a long winded all-in-one Q&A session (if anybody is kind enough to fill in the A part for me)

1). Most noticeable with OD off and PWR mode on, between 37-40mph (doesnt matter what rpm or how wide the throttle is open) I can feel a vibration and hear a low, pretty quiet rumbling noise. Seems like its coming from the engine maybe. If I cruise at 37-40 it's there the whole time. If I speed up from 35-45 I only notice it going thru 37-40mph, same goes if im dropping speed. With OD on it's still there but only about half as noticeable. With normal mode its almost not there at all. WTDump is this?

2). Why does my car roll/cruise better with overdrive on? If I speed up and let go of the gas entirely, with PWR on and OD on I can let my car cruise effortlessly (downhill or across a flat area), similar to my BMW 740 (both cars are damn heavy and cruise great hehe), but if I leave PWR on and turn OFF the OD, then I lose speed when going across a flat area. It's especially noticeable when going down a sharp hill - I will not gain speed AT ALL. When going across that flat area I'll lose speed much more quickly than when I have OD on. Also, with OD off, when I cruise and lose speed I can hear what sounds like tire noise (my yokos have like 50% tread life left), but it's not as evident with OD on.

I know this was pretty long and these are probably some whacky problems. I was wondering if these are minor or If I would need to do something major like a tuneup, or a engine/tranny rebuild. Any help is appreciated!