[CrazyBRONCOguy]

Keep it clean is the biggest thing.

Yes a valve body can be taken out of a c6 without removal. It's is not very hard. Pay attention on install to make sure the gear selctor and valve are lined up.

For what it's worth I have had sticky valves in a c6 make this happen too. New valve body did fix it.

It's also a good time to adjust the band

 

[CrazyBRONCOguy]

Foreign Objects.

That is worse then any that I have seen. I agree the governor maybe stuck. If you do, do a new valve body I would expect it to just "maybe" work. You could do a full manual valve body and a floor shifter and drive it like a 4spd (kind of). That is what I do with mine.
I would for sure add an external filter to try and clean up what you can that way too.

If was mine I would likely be planning for a rebuild and be surprised if I swapped the valve body and it fixed it. I feel like I would still try tho if I had it apart that far. You may spend sometime cleaning what you have up, or at least take the side plate off and inspect the valves.
Broader Performance sells nice valve bodies if you cannot come up with a "known good" one

 

[CrazyBRONCOguy]

Takes a special valve body, then just a non column shifter like a b&m megashiter or a winter's (there are others too) The manual valve body doesn't need any input from the governor or the vacuum modulator. If your current governor is stuck it's already acting somewhat like a manual VB would. I have pics in build thread but can post more here. This may be can or worms or rabbit hole discussion of what your actually having issues with tho

 

[CrazyBRONCOguy]

As far as I know you can mix and match most C6 valve bodies without issues. The exception being that the early cases have the gear detent setup a bit differently. The early ones use a spring and ball setup on the valve body itself, while the later ones use a "rooster comb" and a spring setup that is on the trans case itself. Should be able to visually confirm this. Your stock bronco will be the later design, but on these old rigs always best to verify as parts can/do get swapped around.