94 Acclaim trans trouble

My Father-in-Law has a 94 Acclaim. He told me that he was having trouble with it stalling. It has the 3.0. I changed the plugs in it, which took care of the stalling problem, but when I was test driving it, I was slowing down for a stop sign and the trans locked up. The wierd thing is that I can push the car if the engine is off, but if the engine is on, I can't push it, and if I try to move it under it's own power, it jumps slightly, but thats it. Do I have to replace the trans?? Thats pretty much where I am at, but if it's a simpler thing, I would appreciate any help.

Pull the wire to the lock up torque converter.