Update
So I thought I would provide an update on this. I hired a tax attorney to provide an opinion. His opinion centered around 2 things:
1) that the wording in the decree was ambiguous enough that it could be interpreted different ways. Here is part of this analysis: "It should be noted that when an ambiguous contractual provision lends itself to two interpretations, a court will not adopt an interpretation that leads to unreasonable results, but instead will adopt the construction that is reasonable and that carries out the purpose of the contractual provision. A court will avoid any interpretation that is commercially unreasonable or contrary to a reasonable interpretation." So my argument here would be that the existing wording does not take into account any stock sales that happened after the divorce or any change of tax withholding or anything of that nature which could drastically change the amount of tax one of us owed...so is really not a reasonable soloution.
2) When there is a situation like this where the parties do not agree on the interpretation in a contract, the court will typically side with the party that did NOT write the decree. In this case, my wife and her lawyer wrote the entire decree.
I have also heard essentially the same thing from the divorce lawyer of a friend of mine. I have sent the letter from the tax attorney to my wife and her lawyer and we will see what happens.