Engine or Transition Problem?? HALP
 

Ok so I have no idea what is going on with my Jimmy, sometimes when I'm driving it will shift into 4th just fine, going 45mph does 1300RPM and 70mph does about 2300 RPM. Then other times (whenever I stop or different times when I drive) it will KIND-OF go into 4th or not at all, so at 45pmh I will be at 2000 RPM and at 70mph it will be at 3000 RPM. What the hell is going on?! I dont drive like a madman, I accelerate at a steady speed, maybe even slower and no matter how I drive it doesnt make a difference, its just random. So if anyone has any idea whatsoever of what could be happening PLEASE HELP ME, I hate my gas bill.

When was the last trans oil & filter change?
Is it over heating because of a clogged up rad or lots of weight in the rear or towing?
Could be excessive drag from low/impropper tire pressure, alignment out.
Larger tires, a lift, roof rack... all will cause drag & that'll kick the trans in & out of OD.
Even my trans that's rebuilt 1000 miles ago with a 2nd stage shift kit, corvette servo etc etc, can handle 450HP & has an aux. cooler has issues when I'm towing a boat on the highway with a head wind.
Mind you I have much larger than stock tires, rims, BL, SL etc. But still wind has a huge effect.
If a trans fluid/filter change hasn't been done in 50,000 miles, Do It! It should be changed at 50K anyway.
Maybe even get them to backflush the trans to remove the fluid in the torque converter, because it holds at least 1/3 of the trans capacity.

If the back flush & new fluid doesn't help maybe try some Lucas Mucas for transmissions. It's a bandage of sorts until you get the $ together for a rebuild ($1300 - $1600 USD give or take, depending who you know) because the seals & belts are worn. A trans shop may be able to tighten up the seals but IMO you may be looking at a rebuild.

New transmission with new oil and filter a year ago.
Don't tow anything.
It still did this even before the lift, I'm running 31s with 4" lift now.
I'll try the flushing and all and see if it helps.