one of the problems they had with those big 4 cylinders was that they did vibrate at lower rpms, is this a new vibration? one that wasn't there before? Or has it been doing it all teh time? A couple of things to check is the front and rear motor mounts. Make sure the rear are tight and the front lags are still secure, check that single isolater on the front mount, make sure the nuts haven't vibrated loose or teh isolater itself is bad. Use a pry bar and see if you can move the engine up and down from the motor mount.
If everything checks out OK, I wpould suspect the coupler, next time you have the drive off, stick the alignment shaft in the coupler, then rotate the engine 90 degress and check it again, do it 2 more times. mercury told me to crank the engine(with a water supply hooked to it) while the alignment tool was still in place, and check it for wobble. you can really fine tune the alignment this way.
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