what I might do : first I'd push the od button after it shifted up , see if it IS the L/U that failed , cause the way the rpm's drop and the truck speeds up , I couldn't tell you which was which , gear or L/U both seem to do a gear ..I have one truck that will pop out that button on bumpy road occasionally / rarely enough I don't care why .
then I would look at the wire diagram , the eemc solenoid wire color . unplug the harness at the trans , SOMEhow connect a lead to the center pin , up thru boot to my test light , I'd connect a 2nd lead to the eemc pin , up thru the boot , drive the truck , warm , above 42 mph , cruse light / throttle , touch test light ends to the 2 test leads , should light test light . remember standing still , engine running , the center pin/wire is positive , the 2 outer pins will be nothing , not ground , not power . when the pcm ( computor ) decides its time for od or l/u it sends ground to the outer pins / wires , so your looking FOR a ground NOT a positive .. if your light lights your harness and pcm control works ...
an option : buy a male connector wire end ( Napa) disco harness from trans plug in your test connector , wire a fuse from ? cigar lighter positive power to center wire , drive truck apply ground to either outter wire see if it shifts . and or locks up , then you know trans /converter works
a pita fer sure , and yes by all means check blink codes first ,, damd good idea !