Can they be re-packed? I had mine re-packed when I had the timing belt changed and brakes done a few years ago. I believe that service cost me about $1,200 for everything. Since then however I had to replace some wheel studs, which basically required me to follow the steps for taking the hub apart and exposing the bearings. I went ahead and re-packed them when I had it apart. Replacement requires four bearings (two for each wheel), two seals and some grease. So the price tag on the parts seems high, but admittedly I have never purchased the whee bearings.
I don't know what type of damage you could do by driving another 500 miles. I came to learn that mine were shot when I bought it and I had not noticed anything, except it was a MUCH smoother ride when I had the work completed. The guy at the shop told me they were really dry and was surprised I had not noticed anything. I guess I was used to a rough ride from the Explorer I was replacing. It's all relative, but I probably put many more than 500 miles on dry bearings and having them re-packed made them good as new. YRMV of course.
In case you are interested, here is a site with the procedure I followed for my work. It's labor intensive, but not too difficult. The site is for an Xterra, but worked for my 99Q.
http://rkrenn.com/xterra/howto/bearing/bearing.htm