Update, I decided to rip up my Mom's existing driveway with the scarifiers and do a little hand raking. Yeah it was more hassle, but her driveway turned out not to be as rocky as mine. I did disturb several melon sized boulders, but too few I think to do any harm. By ripping up the old driveway, before spreading new gravel, it feels to me I'm doing a more complete job. I don't have any experience in this line of work to substantiate that perception, its just my personal opinion and it was bugging me.
Here are some before and after pics. I hope to start spreading new crushed gravel on it in the next day or so. Also included is in the pics is a "harrow" I made from an old clothesline post and some old tire chains I found in the barn.
Now that I have the tractor and equipment, I plan to do annual maintenance on the driveways and not be as aggressive with the scarifiers.