Servicing the differential at 18,000 miles?

I have a Chevrolet Silverado with 115,000 miles on it and I have never serviced the differentials, there is a check hole on each differentiall (the front and rear) and it is removed with a 3/8 drive ratcher. Stick your finger in the hole after removing the plug (located on the side of the differential NOT THE BOTTOM) the bottom plug is the drain. If you can feel fluid with your finger, you have enough fluid in your differential. Also, if the differentials aren't wet on the outside, they most likely aren't leaking. I would not suggest servcing the differentials.

Tell your shop to look up Technical Service Bulletin (TSB)
#01-07-30-030B

That Bulletin is specific to your problem.

NONE of the 4 listed conditions in the TSB have anything to do with your differential. Your mechanic is an idiot...if the problem happens on one shift but no other shifts, how could the problem even remotely possibly have to do with the differential, which has to handle power through every shift?.....Yea, thats right, it can't have anything to do with your differential.

If your problem is only a metallic clunk noise, but not a hard shift feeling, this is actually normal on gm trucks with a 4L60-e or 4L65-e transmission and is of no cause for concern.

Good luck hope this helps