Why does my transmission shift so hard when i hit about 30, it stops the tires.

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When I take off my van shifts just fine until i hit about 30 mph, then the front tires actually stop for a second then it shifts and its ok, until I stop and have to go through it all again, we already changed half the transmission fluid, afraid if we changed all of it and the filter I'd have a complete transmission to replace because of it's age, already put a new shifting module (i think that's what its called), changed the fuse under the hood, I don't know what else to have done, as long as I go easy and slow to reach 30 mph its not as bad and first time running it after its sat overnight, is good, but after I stop the first time then it gets bad. please help me, I just bought this van, used of course, but it runs so good other than this problem, it has the 3.3 engine in it and is front wheel drive if that helps at all.