Neglected U-joints can lead to eventual failure. Typically, as a joint wears it will go from being well lubed and quiet to a drying joint that may give you warning by starting to squeak. Beyond this point, the joint wears metal-on-metal, and it will begin to develop looseness between the cross and the caps. At this point, a joint can come apart, leading to some serious drivetrain damage when it lets go. A quick way to check for joint play while on a vehicle is to put the axle on jack stands with the transmission in neutral and parking brake applied. Then simply grasp and wiggle up, down and rotationally, looking for any signs of looseness in the bearing caps (and in the splines while you’re at it).