Squall and slippage under pressure?

I recently purchased the vehicle listed above with 42rle transmission. I didn't notice until later that there was a loud squalling sound, shudder and slipping when taking off hurriedly, going up a hill, or underload while in first gear. It stops when the accelerator is depressed but resumes if pressed firmly again until it changes to second in which it does fine. It seems as if anything that puts pressure to the drive train will cause the symptoms. It also sounds like it's ready to shift the entire time it's in first gear. I came to think the problem was the torque converter, so I purchased a new converter, filter kit, seal, and fluid, then had my mechanic do the change. I added a bottle of transtune afterwards as requested by my mechanic. I got the truck back the day before yesterday and didn't put it under any pressure till yesterday and the same symptoms resumed. I've been researching the problems again with little success. I've spent a good bit of money changing the torque converter and filter, so I'm hoping there's a not so costly fix. Does anybody have a suggestion? Thank in advance!