The universal joint is worn or damaged.

Replace the complete propeller shaft. Refer to Propeller Shaft Replacement.

The universal joint has lost lubricant.

Replace the complete propeller shaft. Refer to Propeller Shaft Replacement.

The rubber coupling to transmission or differential flange retaining bolts and/or nuts are loose.

Tighten the bolts to specifications. Refer to Fastener Tightening Specifications.