Craig,
It sounds like what you are describing is the need for a software that focuses more on management of the actual CAD models and related documents. In my experience this is typically termed "Product Life Cycle Management", or PLM. An ERP system is a bit more focused on the transactional nature of what a company produces and focuses more on customers of that company, as opposed to a PLM's role which is more inline with the engineering and manufacturing departments of a company for part design life cycle maintenance.
I work for a company that helps clients implement, host, optimize and monitor PLM software, with a particular strength in PTC's Windchill PDMLink/ProjectLink solutions. I can vouch that PDMLink is definitely one of the best tools out there for engineering/manufacturing design management, and I think if you ask around with others in this forum you will find that to be true.
Although I am not a direct end-user of Windchill, I'd be happy to answer any questions you may have about PDMLink/ProjectLink if you are looking for more details, and FYI I live only a few hours drive away from your location, in a suburb of Chicago. Please let me know if there's any way I can be of assistance to you.
