1999 chrysler town and country driver's seat moves fore and aft when braking or accelarating. What's the fix for this?

Move your seat, while sitting in it, until you feel a click. If you do not feel a click, look under the seat to see what is preventing it from locking in place. Hope this helps

My cousins seat did this in her older truck. The piece that would click to hold it in place under the seat had broke. I would suggest inspecting under the seat after you tried the previous answer. If it is broken, you may need to take it to a dealership or pull a new seat from a junk yard. Bolt it down indefinitely in one position.

Would like to thank you both for taking the time to answer! I removed the side and front shields for the seat. There's a black cylinder with a white plastic gear sticking out that was just laying there. i stuck that into what appears it should go and the seat was ok until I moved it. Unfortunately the cheap metal casing that attaches the cylinder to the other part is broken. That appears to be the problem. I sorta fixed the fore and aft motion by using a bungie cord. I wrapped the cord around the bottom frame of the seat to the round bar under it. I still get a slight aft motion when I accelerate but no fore motion when I brake. I don't know what that particular part is called but it appears to be replaceable. However, I may need a special tool to do that since the bolts are not hex nor torx nor anything common. I will be visiting the local junk yard. I will be looking for a manual seat like the passenger side. Thanks for your time.