transmission fluid question?

My guess is that you don't have enough tranny fluid.

The ONLY thing I can think of is this:

I have to ask if you checked the fluid level on a level ground, engine running, in park? My guess is you didn't. When you removed the excess fluid, you removed way too much; now your fluid level is too low by 4 pints. The fluid level on the dipstick is one pint from "Add" to "Full". Excess fluid can't compensate for anything.

Looks like it needs the extra fluid, for whatever reason. Ran fine with it and runs crappy without it, so put it back. Double check you checked the fluid level the right way, engine idling and transmission warmed up, shift slowly through all the gears, then put transmission either in neutral or park- whatever is called for, then check the stick. Maybe the wrong dipstick is in it. Go get you another one at the salvage yard and measure it to compare.

I'd be more inclined to think you don't know how to check the ATF level.

Was the motor at operating temperature?

Did you check the level while the motor was running as required?

did you pull the ATF dipstick and wipe it off, re-insert it and then pull it out to check the level?

When you mess with a vehicle and it gets worse usually the person working on the vehicle is the problem.

You just need to make sure you check the tranny fluid while the engine is hot and running. It looks too high when the engine is off and cold, it looks too low when the engine is running but cold. Hot and running, then check it and add as needed. The only thing that usually occurs in the old school Chevy/GM trannies if you over filled them, was some fluid might come out of the dipstick tube or the breather tube on top of the tranny housing. That's about it. There's no "extra" pressure on anything if you overfill the tranny. It's bathed in fluid, top to bottom with really no air inside. Different story if you accidentally dumped an oil change worth of motor oil on top of a full oil pan.

I have exactly the same truck but 4wd. It's probably a trashed torque convertor. Transmission rebuilds for that truck run $1300-$2000. Mine cost me $1700. Boy howdy did you get hosed. Learn how to do a proper pre-buy / pre-trade inspection next time. EPIC FAIL.

it is difficult to tell what is going on. You are checking the transmission fluid with the engine running I hope! If you check it with the engine off, it will read high!