The fluid from the transmission does "migrate" between the transmission and the the u-joint "ball between the transmission and the enclosed driveshaft.

There is a seal at the rear of the u-joint cover - the torque-tube housing can slide (a little) backwards and forwards as the rear-end moves up and down on thew springs.

1954 truck Shop Manual

universal joint ball/seal/gaskets adjustment procedure
gasket/seal set (the seal is in this set)