without a tach you can look up the gear ratios for your trans and rear and figure what the engine speed will be for a given mph. You should drive it and note when you have vibrations, noting also if it is in all gears. Then figure what rpm's the engine is doing at the vibration points. You should also bring it up to vibration speed, and ease of the throttle, and see if the vibration goes away. If so lugging it well could be. I do not think lugging is your problem though. I would suspect a driveline vibration, or engine imbalance. Was this engine mated to a automatic? If so then it is likely the flexplate may not have had the same balance as the flywheel you are now using.

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1977 240D turbo