Double cardan joints are a type of universal joint often used in applications calling for more articulation and torque-transfer capability than a standard Rzeppa joint can provide. While far tougher than most other types of CV joint, the DCJ will still wear out and fail when subjected to enough stress and wear.
Clunking under acceleration or sudden braking is often the first sign of DCJ wear, and can presage an immediate and catastrophic failure if left unremedied. This clunking comes from excess bearing play between the DCJ's individual U-joints and its intermediate shaft, or inside the U-joint caps themselves. Clunking is dangerous because the clearances it implies allow the driveshaft to build up momentum before engaging the wheel, which hammers the bearings to slurry.
Binding may happen before audible noise in the DCJ, but it often occurs as a result of excess bearing clearance in the DCJ's U-joints. Binding generally happens under extreme suspension articulation, and manifests as a very expensive-sound popping noise. When the front DCJ on a 4WD truck binds, it might send a noticeable shimmy up the steering wheel and jerk it slightly to one side or the other. Enough binding will eventually break the DCJ's U-joints or intermediate shaft, and even mild binding can crack it enough to cause failure later.
Vibration in a DCJ is fairly rare, since one side of the joint usually mounts to a solid base, but it is a sign of impending failure. Vibration will typically occur only on extremely lifted or lowered trucks where the DCJ never fully straightens out under cruise conditions. While few CV joints ever operate in a fully "straight" position under cruise, sustained angles will allow the intermediate shaft to oscillate in the space provided by the U-joint's bearing clearances. As such, vibration almost always heralds severe bearing wear, and will probably result in binding if left unattended. Bear in mind, though, that DCJs will always vibrate a little more than other types of CV joint, so don't replace the joint unless you detect a noticeable increase in vibration.
Checking the DCJ
The simplest way to check a DCJ is to crawl under the truck, grasp the driveshaft in both hands and try to twist or move it. Since most DCJ failures are a result of bearing wear, any excess movement in the unit will tell you that something's wrong. Allowable clearances vary, but excess wear is probable if you can rotate the shaft enough to note an audible tap when it stops. This mild tap will eventually hammer the bearings into a full-on clunk, and then you've got problems. If you're replacing a severely worn U-joint in the DCJ assembly, then wash the intermediate shaft and yokes with degreaser and carefully inspect them for cracks. You'll typically find cracks in the thin area between the U-joint cap holes and the edge of the yoke or intermediate shaft.