Welcome to AF!
Sounds like your trans might have been hot.
At 50k miles it is time for a fluid and filter change, no flushing !
It isnt recommended to drop it to lower gear at higher speeds. Though I doubt 2-3 times would kill your trans with only 50k miles.
I regularly down shift from OD into 3 or even 2(but I can replace/repair damages incured due to my abuse lol).
I tend to do it most when I'm speeding, and I think I see the 5.0 clocking people.
If your service lights are still on , you'd need to get it scanned with an ABS/Trans capable scan tool. Most of those free scans at part stores dont show these type of trouble codes.
And to check the level of these tranys you need it on a lift, remove the check plug on the pass side of the trans, if it runs out it full, if not its low.
Realy crazy not to put a real dip stick to check your fluids.
Hope that helps