shuttering

I'm suspicious of a torque converter clutch (TCC) problem causing the problem due to incomplete engagement under moderate power in a high gear, which would cease if you let off the gas or accelerate because the TCC would disengage under those conditions. Also, there is a possibility of the transmission clutches or bands beginning to slip and shuddering on application of moderate power, also ceasing when you accelerate (downshifting?) or let off the gas. I wouldn't suspect the motor mounts from the way you describe the problem. If you want to find a specialist, here's a directory link for you: https://repairpal.com/mercury-repair-in-pittsview-alabama

I would make sure that your engine is running properly, that it is not misfiring etc. Is the any check engine or service engine soon light on or blinking? That may indicate a misfire condition and could/would cause a 'stuttering' under acceleration.