I understand what you are saying.. But I do want to point out to you that you have a 4T60-E... Being that this is the trans you have the Diff housing is only held on by 4 bolts, and the Diff is simply in place with a C clip.. Once the diff housing and C clip are pulled the diff will come right out and can be replaced with another.. The only other possibility or issue I can think of is if the sun gear has been damaged or destroyed as well..
Yes fight for a better warranty... Get the $2200 in writing so that they have to go by their quote...
It is rare to see a Diff pin snap unless you were possibly in some snow and one tire spun too much, In this case the diff pin will suffer a lack of lubrication and get cooked, it will either sheer or lock up and snap..
I wish you the best on the rebuild, and let us know how it turns out
