Fuller 9 speed

Detent springs in place?

 

After that it is usually the sliding clutch and gear worn down. I thank the purple detent spring is the strongest. One of those might buy you a little time.
Have to knock the top off and look inside.

 

Be sure the shift tower and O ring in top of it are good.

 

That's a great point.

@truckerbangin, can you describe "slips out of gear"? Are you having to remove the shift tower to get back in place?

 

Jumping out of every gear, or just one?

 

I know you said the sluding clutches looked good. Have you seen a worn slider? An unknowing eye might think it ok.

You can see in this pic the sharp edges of the slider. Were yours much more rounded?