I had a similar problem on my 94. Not sure if you have vacuum controls like my 94. but if you do, I took it all apart cleaned and greased up the controls. They were all dry and gunked up. Cleaned and checked all the vaccum hoses and connections. Worked great after that. I just have to remember to pick up a new knob from the junkyard next time I am there. I had been reefing on mine when it was not working properly and broke it. Now the controls actually click at each setting and are very smooth.
I can't recall all the details but there are little detent ball bearing thingys that run in a track and fall into a groove when you move the control Knob and they make a clicking sound when running smooth. My other vague memory is there is a round rubber plate like thing that has a ridged pattern on one side of it that directs some of the vaccum pressure as you turn it. I think it was on the defrost control that you were having problems with. Mine was gunked up and not working properly. Every thing worked after I cleaned everything and greased the Nylon parts that looked like they needed lubrication. Sorry about the vague memories and descriptions, but if you take everything apart you should be able to see how it all works and clean and reset it to original unless you find broken parts inside.
94 ZJ,4.0, AX15, NP231