Yep, the flutter is a common problem because that is also a common speed in slow traffic or small towns.
As Jack mentioned, I have also turned all my shift points to the minimum to keep RPMs down in stop and go traffic. When I approach the shift zones I'll just drop the shifter into 2nd or hit the O/D off button to drop down into 3rd. That way the truck avoids unnecessary shifting.
Pictures are worth 1,000 words and this is handy to remember when you are adjusting shift/lock points.
Feels weird to think I made this two years ago for a very similar problem...but for a guy way up north (north Texas that is
).