shudder on hard acceleration

[The Walrus]

It's been a bit since I posted, I need some more advice.

My car *still* shudders on hard acceleration, and it's funny because it
smoothes out when a constant speed is attained. My suspicion is the
half-shaft axle - the drivers side one is still stock to my knowledge - I
replaced the passenger side one with a rebuilt NAPA one. The engine mounts
are fine, but when I'm at a constant speed, let's say, 75mph in fifth gear,
the shifter shakes like a mo-fo - so I think maybe the mounts are just
absorbing the vibration at that speed, and on acceleration, the whole car
feels it.  AMAZINGLY the engine mounts still seem to be solid, no major
cracks/splits/tears/whatevers. has anyone ever gotten a bad half-shaft from
NAPA? is it possible that that part is defective? is there something else
I'm not looking at? bearings, ball joint, steering knuckle, toilet duck?

Thanks for the input, I know I posted before, but the answers weren't the
problems, and now I have more information.

Dan Landberg
Southern, ME
Rust Free Superblack 91' NX2K

P.S. is it me, or is the NX2K more rust resistant than the B13 Sentra?