Hey Scotty Love your channel,I
learned a LOT from your videos.I have a 1999 Chevy tahoe and one day a while back reverse would not work when parked on a slight slope and had to put it in drive to leave.It hasn't had any issues since that day.when I got it there was 143,000 miles now it is at 165,000miles and haven't changed trans fluid since I owned it in 2010.could the fluid be the issue here?

Also what do you think about upgrading in the future to the 4L80E transmission?

I doubt it, I would upgrade the tranny when it goes out if you want to keep it