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there are many possibilities as to why a transmission will shift "hard" between gears. Starting with the simplist, the fluid might just need to be flushed and changed. A valve in the accumulator valve body could be sticking either from dirt, scoring, or even broken or tangled up springs behind that valve.
I believe it is normal for the transmission to run in OD all the way down to 35mph, as long as when you give it a good bit of go, it downshifts normally.
by the way, when i say OD i mean fourth gear. 2001's only have 4 speed autos, the torqueshift wasn't available yet.
Does it do this when its cold, because you don't get TC lockup in fourth until the tranny gets to about 140* or so. It will certainly feel hard when it does this, more so at light throttle.