......................Pull a Nylon stringline and stretch it from the first post too the last post . Do this by driving a nail in the center of the post on TOP and stretch too the last post . This will give you a line too Align all the other posts off of . Now , I obviously can't see your fence so I have no way of knowing how far out of kilter the first and last post(in the 150 foot span you referred too)......ARE . From this stringline I would try and remove and reset each post . Judging from your description I'm thinking cement is the only way too anchor the posts in the ground . If the fence is made up from panels you can try and detach each panel , clean out the hole , maybe deepen a bit and cement it back into place . I'd also want the bottom of the panels off the ground a couple of inches too keep them rotting away . Lots of work but worthwhile if you have the time and energy . , fordy