Agree.
Google has absolutely nothing to do with this. Google is actually the way you could resolve this.
To turn this into action, probably the most efficient way to work on this is to provide your current happy customers with a testimonials page on your current corporate blog.
To collect feedbacks from your current clients, do a survey by email (ex: surveymonkey.com) and provide a discount on your service or a special offer for all clients that fill in the survey (even the angry ones). No matter what, this survey will give you analytic data on how your clients percieve your services. Listen to your clients, there might be something you're missing out on too.
Focus on facts! You said that 80% of what was said is untrue, then you clearly need to demonstrate on your website that your not only provide great service to "counter argue" the 80%, but you also need to show that the 20% that was true is something of the past and that you resolved (with facts) the issue. Do it in a professional manner. It's all about making the first good impression for new potential clients.
Also you said you have 90 employees, try to give them a way to express their great experience working in your firm. Provide your employee the possibily to write a blog a week, or even better create a subdomain per department and provide public news (press release).
Google any firm name, if you really look hard for it, you will probably find unhappy customers saying how much company x or y sucks. This is not uncommon.
You can -in conjuction- always have your lawfirm work on this, but dont forget to Focus your resources on providing a voice to the happy customers of yours (and employees).