DESPERATE: I need a transmission EXPERT

Well at full throttle it only shifts when you let off the throttle.

 

sounds vacuum related

 

If it's the same with the 1-2 and 2-3 shift i asume it's not getting enough oil pressure from the governor to operate the shift valve's.
Some possible causes:
Leaking governor pipes
Plugged governor filter
Worn out governor shaft/bore
Governor valve sticking

 

At full throtlle there won't be any vacuum!
And because of that it will have maximum modulator oil pressure workin against governor pressure from moving the shift valve, and so delay the shift.
In this case, when you let off the throttle vacuum will come back and modulator oil pressure drops.
Governor pressure is than able to push the shift valve against spring pressure only and it will shift.