Is it my TRANSMISSION?

when driving my 2002 Dodge Caravan 35 mph or more and letting it decellerate to 30 mph shifting down gears, it makes a clunck and then a jerk. Do you think this is my transmission and if so would it be something that may need the whole transmisson replaced? How much would that cost?

Is there enough fluid in it?

It's definitely your transmission. That clunking/jerking is way too familiar for me. I learned a few valuable lessons with my 1995-97 chrysler transmission (those were the worst years for dodge/chrysler transmissions). Here's what you should know in brief:

1. The most important thing: make sure the fluid is of the right type, ONLY CHRYSLER ATF can go in there. A different fluid will destroy your transmission in a few weeks.
2. Make sure it's not overfilled or underfilled. Check the fluid when the car is running and when it's hot. Shift through all the gears, put it back in park, and make sure you're on level ground.
3. If the fluid has bubbles, it's either overfilled or you have a bust booster valve. If the fluid smells like it's burnt or metal dust, then you need to replace it and it's an indication of a problem.
4. Change the fluid at regular intervals of 60000 Km. This is probably another important step that I came across.
5. DONT take your car to the chrysler dealership. Those guys are either morons, or they are told to lie and cheat the customer. Go to a moderately professional looking shop or a mechanic you trust and has fair prices. Generally, rebuilding your own tranmission is better than a used one, but it's more expensive. however, it's just the luck of the draw on how long it will last.

Let me just tell you a story about the chrysler dealership. I think I've told this story about a hundred times. I took my car in after having two transmissions replaced in the car throughout its life span, and I was due for a third replacement. I told them specifically and wrote on the report "Transmission is jerking, clunking, and slipping. This is the third transmission in this vehicle. Why does it keep doing this?". So I figure that the dealership has advanced equipment to test all transmission related sensors, and should know their own cars better than anyone else. To make a long story short, I ended up paying $200 for a report that said in BIG WRITING: "TRANNY NOISY". I tried arguing that when I brought the car in, I indicated that the transmission was going to die, and that this report is a slap to the face. Funny thing is, I take it to a small shop that didn't even have to pull out a diagnostic tool to tell me what was wrong. Chrysler vehicles have moody sensors that make their cars more complex than they should be.