99 chevy blazer 4l60e 3100 rpm @70mph
 

i have a 1999 chevy blazer 4x4 4l60e tranny. when i do 70 on the highway cold ill do about 2100 to 2300 rpm but when it gets warmed up it will be at 3000 rpm doing 70 and 10-12 mpg's. if i let off the gas peddle the rpms will go down to 2100 to 2200 rpm but to keep constant speed at 70 ill press on the gas and right back up to 3000 rpm and then it will start accelerating again back to 70 then stay at 3100 rpm. i am wondering what would be the problem any input is very appreciated. and if you want or need me to take a video of what i mean i can do it just ask.

torque converter not locking ?

no its locking did the old gas n brake method

Sounds like the 3/4 clutch pack is toast.

How is it up a hill?

When it is coasting down a hill it will be at the rpms it should be at but when it need to accelerate or just to stay up to speed on cruise it will go from 2100 straight a higher rpm then start accelerating or man tin speen

That is the stall. When the converter can no longer slip enough it will feel 'locked' and stall the engine.

However when the converter "locks" it has a piston inside that couples the turbine and the stator so there is no slip at highway speeds(thus increasing gas mileage). This usually dosn't happen until about 45mph and is controlled by the PCM. You should be able to count 4 shifts.
1-2
2-3
3-4
4-converter lockup

Lockup is also temperature dependant. If the fluid temperature is to low... below ~84*F the converter will not lock either.

In my experience 3rd + 4th go at the same time. You would be at more than 3100rpm not matter what temperature. The fact that it does it when it's warm would make me think it is the TCC solonoid or else you may be a bit low on fluid. I've had mine a quart low and it drove fine except the converter had to struggle to lock.

So what do I need to do?

What are the odds that it could be the transmission oil I looked at it and it is quite brown and burnt.

Well with the information about it being burnt and brown, I would do a fluid and filter change. Make sure when you drop the pan, check for sludge along the bottom of the pan. Make sure you get it all cleaned out of the pan. If there is a lot sludge in it, a bottle of Lucas AT fixer mixed in with the new fluid can't hurt

Would that be worth it because i don't feel like spending the money for a flush and it all cleaned out and find out i need a whole rebuild money is tight right now lol.