No problem asking a question when you don't know. That's how we all learn, even though some of us won't admit it. I'll try to keep this very basic and simple.
The 98 & 99 UZJ100 had a "two pinion" design. The picture below shows the two "pinion gears" circled in RED and the side gears circled in blue. The side gears are splined and attach to the axles, and the pinion gears (or spider gears) ride on the pinion shaft and spin allowing the the tires to turn at different speeds when you go around a corner. If one wheel is in the loose stuff, the pinion gears are sending all the power to the spinning wheel. If the spinning wheel suddenly catches traction, all that power is sent back to the other side via the pinion gears. That's where things go terribly wrong and you hear things go snap, crackle, pop.
The "four pinion" style in the 2000 and up have a cross instead of a shaft and four pinion gears instead of two to absorb and transfer the power back to the proper wheel. Hope this helps, and your not too scared of the two pinion if you just use your rig in "normal conditions". Mine let loose under extreme rocking trying to get out of snow with ice underneath. The four pinion replacement has proven itself on the trail in loose rock, snow, ice, and mud.
If your planning on serious offroading, look into an ARB Air Locker to replace the two pinion carrier in the diff, and you won't have any problem then.