The two possibilities the other guys listed are correct. Either te kickdown cable is out of adjustment, or you need the new check valve ball in the trans. Obviously, the simpler of the two is adjusting the kickdown cable, sotry that first. Gene said the cable might be too loose. I just changed my kickdown cable about a month ago and remember it being the other way around, that is, a tighter cable gave me harder shifts. In either event, it's easy to adjust.

There's a little crimp bushing on the kickdown cable that at idle (no pressure on the accelerator) should be about 0.040" from the kickdown cable sheath. Have someone hop into the car and press the accelerator to the floor (with the engine off of course)and check the distance between the bushing and the end of the kickdown cable sheath. As I remember, it should be about 2.00" give or take a couple of thousandths. Use a 13mm wrench to loosen the nuts holding the cable in place and adjust as needed. Even if the cable measurements seem to be correct, play with the adjustment and see if it gets any better. It won't hurt anything.

Also, make sure the cable isn't binding. If it is, you may get lucky squirting some WD40 or some other cable lube into the sheath to free it. Otherwise, it's time to replace it.