How well does the 3rd gear clutch hold?
See, in reverse,the L/R band and the Hi-drum are engaged.
In manual low, it's the L/R band and forward clutch.
In 3rd/Hi, the hi-drum and Forward clutch again.
In 2nd it's the KD band and the forward clutch.
So to be driving forward in selector lever Reverse, It would seem the hi-drum is not engaging, and that would lead to a faulty Drive power transmission.It could also be that the forward clutch is jammed up. Then reverse with a slipping hi-drum,is the same as manual low. And if the forward clutch is jammed up, the forward gears might all still work fine, as long as the forward clutch stays locked on.
If the L/R band fails to apply, then reverse would fail completely, but manual low would still work through the sprag and you might never know the band was done, except for the no-reverse action.
So that leads to the question; how is it in drive, will the hi-drum accept full power. If yes, then I would suspect a jammed up forward clutch, or a faulty valve body, and that could be proven with an air-pressure test. Well not really; the pressure test will prove the clutches and servos are or are-not working. If they are working properly, then by process of elimination, the issue has to be in the VB.
At least that's my thinking.
So If the road test proves the hi-drum is working, and no Neutral-creep,I would be doing an air-pressure test,followed by a band adjustment, probably a second air-test, and finally, probably a VB inspection.