Sorry for your troubles. Provided the fluid was filled per the manual, and solenoids are good, then I would look at wiring. I know you swapped the trans, but my guestinate is that 95% of 6hp26 transmissions on the road have one or more bad solenoids. Solenoid life is only 45k or so on them. Past that i would look at the warness. What are the codes?

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E53 X5 4.8is-Dinan S2-Street Brawler
E70 X5M-The stoplight shocker
AMG E55K-539 RWHP, 629RWTQ-my toy
GX470-the unbreakable truck.
F30 328i-JB4 STG 2. Daily MPG Work Beater
02-WS6, 6sp my other toy
...other stuff parked around here somewhere