As Mrs. Nightbreed said sounds like a rear end. If you do this project you will need bearing/seal pullers, and a special socket to get the pinion gear out. While your at it you should replace all of the oil seals and bearings. Might have to replace the rear axle too. I know on QT's bike the splines on the axle and inside the Ring Gear were stripped, but the gears themself were fine.
Your looking at about $800-$1000 if you do it yourself and you can get the parts.
Its not hard just a PITA, I helped QT rebuild her rear end!!!
If you have all of the parts it will take about 6-8hrs from disassemble to reassembly.
Hint if you do it yourself: Put the bearings in a deep freezer if they are going inside a race (ie Needle bearing for the end of the pinion gear), they will drop right in with little to no persuasion.
If you can find a whole rear end it might work out to be about the same price or cheaper then rebuilding the old one something else we learned.