How can you disable the automatic door locks on a 2000 Plymouth Neon?

Yank the fuse for that circuit.

ok ...without disabling the electric locks.

You can go to your local Dodge dealership and they can change that setting. Some dealers will do it once at no charge, but I would expect a $30 charge to be the norm. I hated that the doors locked after hitting the pre-set speed. My kids are old enough now that I didn't need that feature. It is a child-safety feature. My dealer charged me $20 Canadian to do it.