I am in this situation, I cleaned up and painted my brake drums a couple of weeks ago and found 2 nails in the drivers side and 3 in the passenger side tyres at the back, they arent new but do have a good 10,000 in them with only being rears. I considered my options, it looks like they have all been in for some considerable time and are well embedded in the tyre, I am keeping an eye on them all the time and have the spare wheel if need be anyway. I think I will just get them replaced in the next couple of months.