The lock ring is important because it keeps the U-joint in place and keeps it from moving. If you have the wrong lock ring then the U-joint could rock back and forth and cause damage to the the yoke. If you have the outside lock ring it will have a circular shape with tangs and be at the top. If you have the inside lock ring then it'll look like a C and be at the bottom.
Upgrade to Thermoplastic
Upgrade to Heavy Duty Problem Solver Design. Includes Thermoplastic Rubber with Provides additional performance and handling benefits.