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Ok, check it all and get back with us, do you know if the engine or transmission mounts have ever been replaced?
I'm not sure on the whole shudder thing, but my engine mounts were worn and at low speeds, it would shudder then thump if I gunned it, but the mounts were very collapsed. Regardless, there are people on here who can help you much more than me lol.
your tail shaft coupling could be slightly buggered for a shudder, mine was the same