Q: the tach will stop working and the motor bumps hard during acceleration on 1990 Ford Thunderbird SC

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in gear but in nuetral the motor does not bump, than the tach will start working and everything is fine
(1) Answer
You can check motor mounts to make sure they are still good but, this sounds like a tranfer case/front differential problem. Need to get vechicle off ground put into gear, and observe unit to see if this the problem.