Unless the body, or liftgate, appears distorted, or out of square, or crooked, it probably is not the fault of the liftgate. The problem lies in the fact that the gas shocks push the gate out of alignment, should the hinge adjustments loosen.
The way I fixed mine, seeing that the gate had "slid downwards", was to open it fully, until the shocks are fully extended, then support the gate with some good means, preferably at both corners, at that height, or just a wee bit more, if the shock(s) can be seen to NOT be fully extended by manually lifting slightly higher. The shocks have internal stops which determine the OPENED height of the gate. After supporting, loosen the hinge screws JUST a BIT, until you feel you can manually slide the gate towards the roofline, through the adjustment range. Usually, there is a dirt build-up present which may give a clue to the original location. Tighten them good, lower slowly, observing location. If good, try latching, then look at "fit" If not close, do procedure over, moving lift appropriately. It came out "right on" my first try!
The "key" is to remove ALL force from the shocks when adjusting. If one or both of the shocks are "shot", then one may extend fully, while the other does not, or BOTH may not extend fully. This condition complicates matters a lot. If that seems to be the case, buy new shocks, install them with gate well-supported, then do your location adjusting. Tricky, but "do-able". imp