1996 Dodge Caravan

I just bought this van and already have some concerns. When it shifts from 1st to 2nd, and 2nd to 3rd, it makes a loud noise (reminds me of an elephant) and shudders slightly. The loudness depends on the rpms (if I let my foot off the gas just as the sound starts it barely makes the noise). But if I continue at the speed it shifts at, it continues to make the noise off and on as if something rotating is making the sound and all I have to do to get it to stop is either accelerate or let my foot off the gas. My dad asked a few people and we were given a few options to try. Change fluid becauuse if even 1 qt of the wrong stuff was put in it can cause shuddering. Flex plate. Or the trans. Just hoping for anymore advice or if someone has had the same problems. Thanks