drivetrain takes control / unable to shift going downhill

have informed dealership...they took no action, would not acknowledge issue. ISSUE: transmission taking control and not allowing truck to shift after automatically downshifting going down a steep grade / mountain travel. at most times coming down a steep grade or mountain roads... could be w/ or w/o cruise, in manual shift, tow...no matter, even in 5th gear, while coasting perfectly at reasonable speed(40-50-speed limit)

the truck (ECM? trainsmission? drive train? ) will downshift and not relinquish the ability shift until the truck reaches flat road... This forces me to have to brake hard to bring it to 25-30 ish mph and creating a road hazard. I am seeking anyone who can point me to someone reliable to speak to on recording incident. going to record by video next trip somewhere where it will happen... but I want to record truck live by OBD in real time...surly someone knows someone... this issue is going to cause serious problem