Truck Repair: DIFFERENTIAL NOISE, limited slip differential, worn bearings

Question
I HAVE A 1995 GMC VAN 2500 SERIES WITH 138,000 MILES.JUST RECENTLY A CHATTERING NOISE HAS COME FROM REAR END AREA.U-JOINT CHECKED OUT O.K.BUT WAS TOLD BY REPAIRMAN THAT GEARS WERE PROBABLY BAD.EXPECT REPAIR COST OF $1200 DOLLARS OR GO TO JUNK YARD AND BY USED REAREND.I CHANGED FLUID AND FOUND THAT FLUID WAS ODOR LADEN WITH SMALL BUBBLES.DID VISUAL ON GEARS AND DID NOT SEE ANYTHING UNUSUAL.REPLACD FLUID WITH PROPER TYPE ALONG WITH REQUIRED ADDITIVE FOR LIMITED SLIP DIFFERENTIAL.SOUND IS STILL PRESENT WITH MORE NOTICE UPON DECELERATION UP TO STOPPING.WILL GEARS LOCK UP EVENTUALLY AND OR CAN I DO REPAIRS MYSELF BEFORE LATTER HAPPENS?                             I APPRECIATE YOUR INPUT-----THANKS,JERRY

Answer
Hi Jerry,
Not enough info to really say.
You should have pried the diff carrier, wiggled the pinion shaft, removed the axle shafts to check the bearings, and check the brakes for cause of the chattering.
Also make sure the tires are the same size, and one is not substantially more worn than the other, which would make the circumferrence different, causing the axles to rotate at different speeds.
Check everywhere for loose or worn bearings, and excess slack.
I would expect it to get very noisy and be slinging oil before the gears lock up.
Now pulling those axle shafts is not too hard, and will give you a chance to check the spyder gears also.
Catch that new oil in a clean pan, and reuse it.
Van