A general statement first - you should not pay for anything before the divorce that you do not want to pay for after the divorce.
The wifey's complaint - that you are spying on her - can be addressed head-on. Her concern for privacy justifies you cutting the family cell plan, and she is welcome to port her number to any old service provider that she wishes. Paying whatever the cost to get her off of a shared plan is going to be minor compared to the expense of legal fees debating how you have been stalking her and "controlling".
The family plan, as with many things, is a perk of the marriage. She can put her big-girl pants on and start to cover her own expenses, as she will have to in the future.
One important aspect to dealing with your known trait of snooping, is to cut the opportunities to do so as you enter the period that matters most. More importantly, how can she deny this independence of her own cell account if she is concerned of living under the microscope. I bet she keeps her phone password protected now, so that you can't access it, out of similar fears.
Make sense? To be certain, this probably extends to internet service at home. Do you share computers?
Volenti non fit injuria