The hole you see on the end of the yoke between the ears (where the U-joint attaches is a vent hole so the yoke can "slip" fore & aft on the Transmission tailshaft.

You can have a dry joint and it will feel ok. Worst thing the manufacturers did was install "sealed" U-JOINTS.....

Joints are relatively CHEAP compared to replacing a shaft with associated costs of balancing and installation.

Go ahead and replace them and make sure they have grease fittings (zerk fittings).

Larry