To the best of my knowledge, you should be able to replace the gasket. (If I am thinking of the same part you described.)
April, 3, 2013 AT 2:19 AM
As you are looking down at the top of the gearbox there is a square plate (for lack of a better term), that has a bolt in the middle that I believe is for adjusting the gear and four bolts on the outside corners to keep it fastened down.
The Chevy dealer says I should replace the whole box; of course they would charge me $800+.
April, 3, 2013 AT 2:22 AM
You can remove that but you may have to make the gasket.
April, 3, 2013 AT 2:28 AM
Once it warms up a little in the frozen northland I will tackle it and see how it turnes out.