Well, the sun gear is inside the low range unit and rides on the output shaft of the transmission; the splines can wear between the shaft and the gear, or be wiped out all together to were the vehicle will only move in low range.
Sometimes the short main shaft inside the low range unit is severely worn as well.
The drive sprockets are what the chain goes around; the differential drive unit is what turns the drive shafts; the splines or cones inside these units can be wiped out as well.
If it's simply the low range shaft and gear you need, you can remove the low range, install the cover, and drive around with high range only until suitable donor parts can be located.
Converting to part time eliminates the differential drive unit if that is what you are asking about.
The conversion has no effect on low range operation.