The carrier is the part the ring gear bolts onto, the big round casting where the spider gears and axle gears mount. The position of the flange where the ring gear bolts is in a different place on the low-geared models, so when you change to a higher ratio, the new ring gear doesn't mesh with the pinion properly unless the carrier is also changed.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
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