Kern,
The drive train in most vehicles have normal backlash, The drive shaft, rear differentials, and transmission connections all have a natural amount of backlash. Backlash is usually present at the connection points where 2 drive components such as a driveshaft to rear differential are connected. This connection usually uses a splined shaft that typically has a certain amount of built in clearances to allow for ease of assembly and the natural vertical and horizontal motion of the drive system during operation.
There is also a small amount of backlash present between the gears in rear end and transmission assemblies. The universal joint connection under normal operating conditions does not have backlash. If the sealed bearings in this component lose their lubricant they will rapidly degrade and begin to vibrate during operation. The noticeable sign this is occurring will result in a strong vibration sensation during normal driving. Most u-joints are sealed and do not require lubrication. However, there are some that have grease fittings which require occasional greasing. I don't believe you have a u-joint problem unless you can detect a vibration during normal driving.