The converter can be rebuilt - I've had a couple more like this since I made this thread. The TCU attempts to lock up the converter at this rpm range, so the shudder is the lockup clutch failing, either because the friction material has worn (this instance) or glazed (a 6hp), or the clutch piston seals are toast (Big Red). I send them to JP Automatics in Bristol who charge about £400 iirc, to open up the converter, fix whatever needs it, and weld it back together. I did hear a rumour that Mackies are cheaper, but JP are local and have never let me down.
I'm not sure what labour to pull the box, swap the converter and put it back in is likely to be. It should be possible in a day for Mackies since they're used to it, but that would only be with an exchange converter. If they need to recon your converter then probably 2 days elapsed.
I'd guesstimate around £1K total.
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