It's a 6 speed manual and when I go through the gears and accelerate it's not smooth especially when I get on it. The car jerks a little bit. Seems to be more prevalent in the first three gears than in the higher gears. The car shifts fine and seems to deliver full power to the wheels. Engine check light is off and no errors reported. I notice it more going up a hill. Engine seems fine. The car has 31K miles on it. I'm wondering if it is the differential or the transmission. Any ideas as to what it could be? Thanks.

Brian