Unfortunately, the factory sealed U-Joints have a tendency to fail. This can occur as early as only a couple hundred miles on the machine. Here are some resources:
"Chatter" in the drivetrain at various speeds
Popping, clunking from the middle of the vehicle
(listed in order of quality, best to worst):
Spicer 5-170-1X Last price $31 at Summit -- The 5-170 will also work, but the -1X has the grease zerk on the outside so it is easily accessible.
Moog 838 Last price $24 at my local O'Rielly's
Moog 338 or equivalent, Last price $20 at my local O'Rielly's
2 seat models need 2 joints, 4 seat models require 3. While the factory joints are sealed, the replacement joints listed here are greaseable. The option 1 and 2 joints feature a zerk on the outside cap. The 338's have a zerk but it is in the middle of the joint and difficult to access so we do not recommend that model except in emergency. Some owners also tack weld the caps on to keep them in.
You can also replace the entire prop shaft if you really want to: 2 seat models 40400-HL6-A01R. 4 seat models have 2 prop shafts 40400-HL7-A21 (front) 40200-HL6-A01 (rear).
Generally, you will need to remove the complete console and unbolt the front diff and slide it forward. For now, here is a video with some helpful info.