Need transmission help

[1984 t-type]

So my 87 GN is having some transmission issues, it’s a 2004R and has a D5 converter. The shift from 1-2 is nice and firm but once you get past first gear and get on the pedal good it will slip right at 4000 RPM, at that point the engine will over rev and pop through the exhaust. If I just cruise in the car and keep the RPM’s under 4000 all is fine. Is it time for a complete rebuild or could this possible be some part that I may be able to replace or adjust ? I don’t know much about transmissions but I am mechanically inclined and do most of my own repairs, I have no problem pulling the trans or pan but if it has to come apart I’ll send it to a pro. Thanks

 

[INEEDAGN]

Pressure gauge on the port abover the shift linkage would tell you a lot of what's going on. If it's freewheeling because it's backshifting into first at heavy throttle you might be able to save it but if it's slipped the clutches or band good and hard even once they're probably glazed and probably means a rebuild for it to ever hold any power.