Vibrating accelerator issues

On my other project s13 I just got, I placed another KA single cam from an auto, to a manual tranny- It runs great, except for that at through low rpms, and at freeway speeds, I can feel & hear a distinct "vibrating" feel on the accelerator that comes & goes.

I replaced my gear oil, and the problem's still there- I'm just curious as to what this may be- any opinions would be mucho appreciated, thanks!

I'd check the u joints