help '89 Ultra has big problem

hey well first off when i drive the car at speeds around 75-100ish km/h(yes im canadian) the car starts to rumble. It only rumbles when i lightly push down the gas peddle, but when i put more juice it doesnt rumble and/or when i let my foot off the gas it doesnt rumble. I have no idea whats causing the rumbling, could it be that something is wrong with my 3rd gear? is trans the problem? By the way the rumbling is felt through out the vehicle, so the rear passengers can slightly feel it.

similarly i think, a few times I gassed the car from a dead stop till it reached 3k rpm and then i let go of the pedal. Once this happened the car went totally retarded, like as i pressed down the gas pedal again the car didnt accelerate, it just revved. What the hell is wrong with the car, it was in overdrive and and not in neutral. COuld it b a transmission prob as well?

tranny issues for sure... get it in for servicem and check the fluid level.

k cool thx