I have a knocking noise from under the seats upon acceleration on my 89xj. It smooths out when throttle is maintained, but come back again when accelerating hard. This surfaced after I installed my 3” lift. everything else is stock accept the upper/lower control arms, track bar, coil springs leaf springs and shocks. I’ve read threads on how motor mounts and the transmission mount can cause this, but they are all new. I’ve also read that u-joints can also cause this same type of noise upon accelerations. My rear driveshaft does has some minimal play at the u-joints when pushing up on the shaft. The front shaft feels pretty solid. Any help will be greatly appreciated.