The trans X I bought a while back (which didn't fix the problem, even with mobil1 syn in the tranny) came in this funky looking plastic container, I think it was about $8 at autozone. The cap is what looked odd, the container almost looked normal. Isn't ~8qts normal?
I bet your problem lays with getting to much vacuum at the tranny at normal throttle. If you were to take the linkage off the IP bleeder valve (white-ish) and run the arm 10mm to the stop it should be from 5"-6.5"hg. It should be 0" at WOT, 15-18" at idle (with linkage on). If you can't adjust the bleeder valve to these values (there is a nut under the cap to adjust them) then you may be on the same page as me, have to fork out $142 for a new one.
What speed does it generally shift into last gear? For all testing purposes I'd say check the EGR control valves for leaks then just disconnect the line going into the cab (w/o breaking that line
) if you suspect leaks there.