Rear driveline vibration

[72cs20]

I've been fighting an annoying driveline vibration for a few weeks now and have not nailed the cause yet.

1966 K20 with a sm465 w/married np205 and a 14 bolt rear axle.

Everything is smooth up to 50 mph, then vibration starts and goes to about 57 mph and then disappears by 60 mph. Happens under acceleration, deceleration and coasting.

I put the rear axle on jack stands and did a run. Vibration present 50-60 mph.

Pinion angle right within spec for a cv type driveshaft.

Driveshaft is new and all u-joints move freely but had it re-balanced. No change.

Wheels and tires removed. No change.

Axle shafts removed. No change.

Disconnected driveshaft from t-case. Vibration gone.

Rear diff is solid, no noises or looseness that I can tell.

However, there is movement at the t-case output yoke. If I grab the driveshaft and move it side to side or up and down, there is noticeable movement at the yoke and output shaft. I did replace the output shaft bearing, but no change. Is this movement normal for an np205?

Could sloppy t-case internals induce a vibration when a driveline load is connected but go away when disconnected?

Any thoughts? Thanks.