TT rumble versus "oscillation": what's the difference?

TT rumble can cause subsonic woofer oscillation which is what you’re observing.

I only get woofer pumping with a handful of warped records in my collection/system. The low cut filter in my Luxman EQ-500 works wonderfully in these rare instances to virtually eliminate cone flap. I believe it starts gradually rolling down output under 50hz and attenuates -14dB at 6hz where the effects of warps are strongest.

Oscillation is any back and forth. What you describe is usually called woofer pumping. The drivers move back and forth, sometimes a lot, but at such a low frequency you don't hear it. Doesn't hurt anything but it does waste amplifier power. 

Rumble is anything from that low frequency on up into audible bass. Rumble usually is reserved for bearing noise, although depending on frequency and level the noise may or may not be audible. 

Whatever you call it, when you have woofers moving back and forth this much it is not always easy to know why. It could be the bearing. It could also be the shelf or rack or floor. 

With an old vintage table the smart move is to first check, clean, and lube the bearing. While you are at it check and clean the belt, motor spindle, and everything the belt touches. Then once you have done this, or if you have a newer table, then move on to eliminate floor-borne vibration from reaching the table. There's a lot of ways to do this, one of the easiest/cheapest being Nobsound springs.