cyclic drivetrain vibration

1970 A-body, late 70s 318 (internally balanced), A500 4 speed overdrive automatic (42RH), lock up converter, 8 3/4 rear.
both overdrive and lock up are actuated with a manual buttons, The driveshaft was recently balanced and driveshaft angles corrected.

Out on the open road in third gear (1:1) I get a cyclic vibration at 1800 to 2500 rpm (45-55 mph). When I hit overdrive it goes away completely. When it gets up to 70-75 mph in OD the rpms are in the same range as where the third gear vibration occurs, but no vibration. Every once in awhile I get up to 55 in third and there is no vibration.

I assume that since the vibration disappears when I shift to overdrive 4th, that eliminates further drive shaft angle issues, or something from the tail shaft back, or it wouldn't disappear.

Any suggestions on what to investigate next???