I hate to admit it, but when mine did this about a month ago it turned out that I had closed a freakin screwdriver in the seam on the right which jammed the liftgate to the bodypanel. Check to see if there's some kind of obstruction, or if the beauty panels have come loose and are sticking.
If there's nothing obstructing the motion then I'd replace the gas springs that provide the counter-force needed to lift the gate. Over time they age and the seal leaks slowly. Once the nitrogen in the cylinder is depleted they will no longer "spring" and can jam internally, which may be the cause of your sticking and clunking when you open it, loss of the pressure in the cylinders themselves will cause the liftgate to "feel" heavier and make it harder to open.
$27 ea (+tax) at AutoZone