1991 4R 3.0 Auto a340h

So when my 4R is cold, 1st gear will slip and shudder the whole truck. The rest of the gears are fine. Once the truck is hot it doesn't do it basically at all. It also doesn't ever do it when in 4LO, even when ice cold.
Yes my fluid is full and clean, fresh filter maybe 30k ago.
It has a new torque converter, didn't change anything.

I'm guessing it probably just needs a full trans rebuild.

My main question is, because it doesn't do it in 4lo, is there any chance it could be a torque converter issue and not a trans issue? I'm assuming the extra torque in low is negating the problem but that doesn't make a lot of sense to me. Then again auto transmission might as well run on magic for all I know about them.