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If you will lay down under the front of the truck and look up over the skid plate towards the center of the differential but to the right a little you will see a plug that a 3/8 rachett end will fit in. Remove it and put finger into hole if you touch oil then its full if not add gear oil until it runs out.
If its coming from the rear differential, add some additive, available from chevy.
how to add oil to 2005 avalanche rear end
On the upper right (passenger for a left hand drive vehicle) side of the differential, there's a plug. IIRC, you need a 3/8" drive ratchet to remove it. That's where you add your diff fluid.
add it through the dipstick tube.
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read the manual
When the fluid falls below the bottom of the filler plug hole it is time to add fluid. Add fluid until the level reaches the bottom of the filler plug hole. In other words, add fluid(while parked on a level surface)until it begins to run out. I have a 99 Tahoe and the manual doesn't list any quantity/capacity for the differential, only how to check the level and refill if needed. Mike
Look at the rear differential housing - you will see two plugs in the housing an upper and a lower. Loosen the upper plug - if fluid runs out of it you have sufficient fluid - if not - add fluid until it just starts running out - then replace plug.
There is a hole in the front diff guard, thats where you add it (remove the nut). On the left side of the diff there is another nut remove it to drain before adding.
remove the 3/8" drive plug on passengerside of the differential housing. just below the structural rub halfway down. add oil to the bottom of the threads.
2" Suspension lifts here: http://www.tuffcountry.com/Replace the torsion bar 'knuckles' and install an add-a-leaf system.