(1) Your first mistake was not taking this to a lawyer before you signed it.
(2) Based on what I found about MI NCAs, MI courts have upheld NCAs that have a 1-2 year limitation. Geographically, how far is this client from your consulting firm? It doesn't sound like they put in a geographic limit. But each contract can be different and the devil will be in the details. I suggest you contact an attorney now and ask.
(3) I don't think it matters that your job doesn't involve "trade secrets". The fact is that you signed a contract and as long as that contract abides by state law/state precedents, it can be enforceable. And it sounds like the company will enforce it based upon the friendly reminder via certified mail.
(4) First of all you need to take the contract to a local attorney who is well versed in MI NCA/employment law.
It is possible that the client company also had to sign something in the consulting agreement that they would not "hire away" the consultants that came to work at their company via the consulting firm.
That said, my husband has been through this same situation but in a different state. The two companies and he finally solved the issue after months of negotiations by the client company (1) paying a finders/recruiting fee of about 3 months salary to the consulting firm AND (2) guaranteeing the IT consulting firm $xxx,xxx worth of work within a certain time period. We didn't hold out much hope at the time that the job change would actually happen and were pleasantly surprised when it did.
Is the client company willing to help you negotiate out of the contract? Have they signed anything from their side? Because if they did, they could be sued for their contractual obligations too.

