I put in 4:10s in January and it is doing the same thing as yours. I get the bucking (minor but annoying) when I let off the gas in 4th below 40mph. I thought it was the tune that was installed in the car at the same time but I think it is the gears. When the gears are installed, an adjustment is made for backlash which keeps the gears etc. tight during deceleration. I think if it is not adjusted properly and there is too much play, we get the lurching while we slow down. To eliminate the other things like intake/tune/engine as a possible sorce of the problem, find a empty road, drive at 40 mph in 4th, turn the ignition off (dont lock the steering wheel), dont put the clutch in and leave it in 4th, and let it slow down. I think if its the rear end, it will still lurch. Im still trying to figure this one out myself. Keep me posted!