2003 Chevrolet Trailblazer Transmission

It's time to call in the professionals. That will save you a lot of time and dollars. The initial mistake was that diagnostic fault codes never say to replace parts or that they're bad. They only indicate the circuit or system that needs further diagnosis. When a part, typically a sensor, is referenced in a fault code, that part is actually the cause of the code about half of the time. The diagnostic time you pay the mechanic for is to perform tests on the wiring and connectors, and to rule out mechanical issues with electrical parts.

"So I replaced the trans with still nothing happening"

"Still nothing" is wide open to interpretation. To avoid confusion, please say specifically what is happening.

This is going to sound stupid, but have you checked if the bolts fell out of the flex plate where they bolt to the torque converter? You should feel something when shifting into reverse, even if nothing moves when you speed up the engine.

What you described can be caused by low fluid level, but you should also hear some buzzing noise from the front pump sucking up air. Try raising the rear wheels off the ground, put the transmission in "park", then see if you can spin the rear wheels by hand. If you can, the transfer case hasn't locked into a range.

I think you misunderstood what I tried to say. With the transmission in "park", the wheels should not spin freely. If you have a locking back axle, commonly referred to as "Posi-traction", the wheels should not turn at all. If they do, the transfer case is in neutral. The most common thing to look at is the shift linkage.

If you have an "open" back axle, when the drive shaft is locked due to the transmission being in "park", one wheel will spin backward when you spin the other one forward. You either need to have a helper hold one wheel from spinning, or you can crawl underneath and try to turn the drive shaft by hand. Normally, when the transmission is not in "park", you can turn the drive shaft relatively easily in the forward direction, and it will turn harder the other way. "Forward" direction is rotating it so the driver's side of the shaft is going down, (counter-clockwise when looking at it from the rear of the truck).

Actually, the direction is irrelevant to this sad story. The point is you should not be able to turn the drive shaft at all when the transmission is in "park", and the transfer case is in one of its ranges.

To move on to the next chapter, my suspicion is you have a Transmission Computer by the right front wheel. Those have a common problem with corrosion inside the computers, and corroded connector terminals. You might want to investigate that first before you do any more with the transmission and transfer case. It isn't very likely you have two bad assemblies causing these problems.