This car (sedan) vibrates noticably from about 8 to 15 mph, under both gentle and hard acceleration. It does not vibrate any other time. I just put a junkyard automatic transmission in this car, and the tailshaft bushing looked and felt OK to me, as did the driveshaft support bushing and bearing (which looked newish). The front u-joint was also OK. (BTW the vibration also happened with the old transmission). Both engine mounts and the transmission mount are brand spankin new. There is no vibration during a stall test (i.e. floor the gas and brake while in D)

So what should I check next? Any tips appreciated!