Thank you for your question.
If the contract was made under Minnesota law in Minnesota, the Court will apply Minnesota law to the case, but that does not necessarily mean the case has to be filed in Minnesota. If the company has enough contacts in Illinois, it can be sued there.
You may have the option of suing in either State or federal court, see
You can get a free consultation from some of the Minnesota pension attorneys listed by location here.
I imagine a Minnesota lawyer in more likely to be familiar with the specific pension plan that you were in than a lawyer in Illinois, but pension law is largely federal law anyway so if you prefer to consult an Illinois attorney, you can find them here.
You need an attorney to read all the relevant documents including correspondence between you and the company and union and the actual pension plan documents. If there is an ambiguity in the pension plan, it is interpreted against the party who caused it (usually the company and/or the union).
Please follow up on this with an attorney.
I hope this information is helpful.