Just an update. Got done with the first two sections today . I'd been too chicken to cut the tops until today. I grabbed the speed square and figured out how tall from the top board the old posts were cut and what angle. Then got the borrowed chain saw and went to work.
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It turns out I was worrying over nothing. The biggest problem was that I tended to drift down as I cut away from me. Not much but enough that I noticed it. This was corrected by twisting my wrist to counter the weight of the saw.
We used a hydraulic CAT auger with a 4" bit to pilot the holes followed by the Shaver HD10 post driver on my Kubota.
The last picture shows the set up as described except with the larger bit on the auger. There were a few tricky spots where the bed rock was only 8 or 9 inches under the soil. The larger bit was used to make room for 80# bags of concrete.
You aren't kidding!! If my memory serves me correctly they can run from about $2k on up. We usually had it on the skid steer but it will work just fine on the tractor.
Well it looks like I'll be replacing another small section in the morning! Someone ran through this fence again yesterday morning, hit and run. It does appear it got their radiator though.