But there's no vibration or hunting when in Neutral!
because there is no load!
Now when your problem starts and you compass is not moving but the gear is on (D) do you feel the vibration? but when you shift to (N) it is gone and the RPM raised a little?
if yes I totally think your problem is the engine!
Check the EGR valve, To check the EGR, remove the vac line to it and plug it off. If the problems go away the EGR is bad or dirty.
symptoms of what appears to be a stuck EGR valve
Rough idling, hesitation, poor performance, weak vacuum and backfiring all seem to indicated that the EGR valve is at least partially stuck open. Additionally, you can hear what sounds to be exhaust notes coming from the open throttle. Lastly, applying vacuum to the EGR valve has no effect on it.
I remember on time one of my trucks after I installed a new CAI I forget a small hose venting hose caused a lot of hunting especially on high gear oh it was frustrating then after the few checking I noticed it and connect it..