Hi,
it is hard to evaluate trees from just a description but from what you sy you may have trees that have some type of borer infestation that has weakened them for those trees I don't think there is much you can do that would be very productive so yo may want to consider removal. for the other trees the best thing that can be done is to make sure that they are growing well and not under stress from hot, dry soils or other things. This makes the trees very attractive to borers that attack weak trees. So if yo can try to encourage good growth through water and in the fal fertility programs.
