Transmission slipping between 2nd and 3rd — fluid level or something worse?

RPM flare on the 2-3 shift in the 6R80 at that mileage is usually worn clutch packs or a failing solenoid in the valve body. Dark red fluid that isn't burnt smelling might just need a change — these should be serviced every 60k miles and many people never do it. Start with a fluid and filter change using Mercon LV (NOT Mercon V — wrong spec). If the flare persists after fresh fluid, the internal clutch packs for 3rd gear are worn and you're looking at a rebuild (,000-3,000) or a reman transmission (,500-3,500).

Try a fluid change first but add a can of Lubegard Instant Shudder Fixx to the new fluid. It's a friction modifier that can sometimes extend the life of worn clutch packs. It's  and it's the same additive transmission shops add to every rebuild. Won't fix a truly failed clutch pack but it can buy time if the wear is marginal. Also make sure you're at the correct fluid level — the 6R80 is very sensitive to being even slightly overfilled.