1997 Toyota 4Runner Noise/Vibration when accelerating

I have a '97 4Runner with 150,000 miles and for the past 6 months I have noticed a low "growling" noise when accelerating or de-accelerating at speeds between 60-70 (previously occurred between 70-80 highway driving). It sounds sort of like the noise a tractor trailer makes when de-accererating down an off-ramp. I have taken it to 3 garages and they refer to it as a vibration in the drive train. They have checked the U-joints and bearings and have greased those parts as well with no improvement. I am now sensing a slight hesitation or sluggishness when accelerating. Also, the check engine light has come on over the past 6 months about 4 times and goes off within a 24-hour period (I have not made it to a garage in time before the light goes out and they tell me they can't check codes without light being on). Do you have any idea what could be causing this? The current mechanic wants to start with taking the tires off and balancing them to see if it is a vibration in the tires.