Google PR is always up and down every 3 months. You can measure it by measuring the type & amount of backlinks it get.
Don't think that tool will be useful. PR is about the site quality that link to your page at present time. Who cares about what happened 2 years a go. Also, google is changing the way is calculating the PR, and it's hard now to obtain a good pr, so you cant compare a pr from 2 years a go with one from today.
There is one tool that record domain PR history. However, you have to be lucky because the tool only collect the domain that has been queried before. If that domain never been queried before so you won't get past info. http://livepr.ezer.com