Purpose of a U-Joint
Universal joints, a.k.a "U-Joints", transmit rotary motion between two shafts that are not in a straight line. U-Joints are commonly found in drivelines and front axle shafts. U-Joints are used in drivelines because the angle of the driveline can change while in motion. Front axle shaft use U-Joints because they have to turn as they rotate.
U-Joints are simple to make and relatively inexpensive to mass produce which is why they are frequently designed to be the weakest link in a front axle assembly, as a U-Joint is much cheaper to replace than an axle. With proper care U-Joints can last for years in a rig you don’t take off road.
Most common types of U Joints are 1310 followed by 1330 and 1350. 1 ton trucks will use 1480 and 1485.
Super Universal Joints are typically made from Chrome-Moly steel (SAE Grade 4340) instead of standard Carbon Steel which improves the strength of the U-Joint by 50% or more. The two major weak links in a standard U-Joint are the cross and the roller bearing caps. In a Super U-Joint the cross is replaced with the Chrome Moly steel alloy while the roller bearings are replaced with caps/bushings or pins.
An Off Road Only U-Joint is a type of Super U-Joint not made for continuous rotation without regular maintenance due to the bushing needing frequent lubrication. Super U-Joints that use bushings are for rigs that need raw strength over long service and are used mostly on trail-oriented rigs.