More importantly why do you have a 904 in a 360 duster? That is just asking for it to get torn to shreds. The 904 was only supposed to be for the /6 and the 318 as far as I am aware. you should have a 727 in it.

I am not aware of anything internally, I am assuming you are talking about the Kick down linkage. It actually just grips the belt to the drum to downshift. You may want to check and see if it is securly attached to the tranny. I am not a transmission expert, But i did tear down my 727(just a big 904) and I cannot think of a reason why the kickdown wouldn't work internally unless your band is too loose or going bad.

Like i said before I am not an expert, but if it were me and I exhausted every resource I had (have you looked at the local library for books on the 904?), I would take it to a local tranny shop with an old guy that knows his stuff. when i put my interceptor transmission in my charger I had one of those guys do it and he did an amazing job with it for only $30.

Oh and if you wanna look at upgrading, i have a 727 that bolts up to a 304 and a case from a small block mopar transmission that i can sell you for cheap :-D I was told It was rebuilt less than 5,000 miles and operational when it was taken out of the jeep, BUT we have all heard that story before. Let me know