My 98.5 3500 4x4 with the nv4500 was acting funny like the clutch didn't wanna disengage and didn't wanna go in any gears so I replaced the slave cylinder and it did the same thing and now it got stuck in reverse and I finally got it out of reverse and now the shifter moves like it's in neutral but when you try starting it the truck trys taking off like it's in gear wheather the clutch is in or not

 

How does your pedal feel?

 

Sounds like a shift fork issue. You can drain the case and take the side covers off to get a pretty good look in the tranny. Will be hard to see the forks but you should be able to see what's jammed up.