Assuming this is not an engine induced problem (mount, engine performance, etc.) then some possibilities could be:
Flexplate/converter imbalance.
Loose bolts/nuts on a transmission mount, bellhousing bolts, etc.
Since rebuilds can mean different things, was your actual transmission rebuilt or was it swapped out on an exchange basis?
If the latter, just wondering if a worn out front pump bushing on a shaky reman unit could be allowing the converter to wobble a bit due to bushing wear and flexplate movement, or flex?