We had something very similar on a Nissan (albeit a T30 X-Trail), but I think that (in front wheel drive mode), the drivetrain is very similar to the Primera of the same era?
It started out, almost like a misfire under acceleration. As the OP says, nothing felt through the steering wheel, and completely absent when on a neutral throttle. This got steadily worse, until (under hard acceleration in a straight line), you'd almost think the engine was going to climb out from under the bonnet!
It turned out to be the inner "tripod" joint on one of the front driveshafts. I don't know if the OP's car has those, rather than the more common CV joints you get on the outboard end of driveshafts, but if there is ANY radial play in that tripod joint, (i.e. grab the driveshaft with one hand and the housing with the other, and try to rotate them against each other as the engine torque would), then that could be the problem. This would have to be done with the weight of the car on its wheels, because if it is jacked-up, the tripod joint rollers will move to a different (unworn) part of the housing, and it will feel fine!
