Truck in question is a 2009 f150 with a 2v 4.6 and 4r75e 2wd.
from the time I bought it the transmission always had a bang downshifting from od when passing and had a supercharger type whine similar to a terminator cobra . It eventually developed a bang shifting in all gears and I eventually took it to a transmission shop in Texarkana TX and after them screwing me around on my warranty through my bank they eventually caved and said the transmission was about to go out after insisting there was nothing wrong. They replaced it with a reman with a warranty on it. It shifted better than it did but not super great but the whine was gone. This was last summer.
here we are a year later and for the last month it's developed a torque converter slip during lock up and also 2nd feels pretty nasty. Sometimes it's very noticeable sometimes it doesn't happen. Best described as a double bump or a slipping engagement. Temps Have been slightly less than half as it's always been. Fluid is fine. It's only been 20k miles since replacement. The kicker is when I changed the oil a month after getting it back I noticed it was pretty dirty to be a reman. There was a red truck same year as mine they confused mine for so no idea what happened if mine got what that truck was supposed to or what, if so they'll likely never admit it. Either way I need to see how to proceed being they screwed me around for 2 weeks last time. There's no codes stored for any tcc problems so kinda lost outside of taking it to them and complaining. It's a damn good truck and it's the only problem I've had in 190k.
what would I look for to be the issue since no codes are present? How should I proceed?

 

Mercon LV gets very dirty fast, this is a known issue. The 4RXX family has been around since the early 90s, it is usually a very reliable trans. That 2wd trans could have gone in a Crown Vic of that year. Torque converter issues happen when friction modifiers break down (e.g. driving in very hot weather, towing heavy, etc). Friction modifiers help the clutches grab. To save fuel, they pulse the torque converter, and if every thing is right it should feel smooth.
It could also be a bad converter, we don't know what they installed, or the quality or the Reman. TC should not have issues in 20K.
A TCC will usually not give codes, but you can monitor it on a scan tool and record the parameters.
We don't know what fluid they used either, Did you do a full flush or a pan drop?

I would suggest lubegard additives (nothing else). If you have shudder, try Instant Shudder Fix. Lubegard Red is also a possibility.

 

Motorcraft friction modifier is best. Genuine Ford Fluid XL-3 Friction Modifier Additive - 4 oz. https://a.co/d/1ycADUh

 

Using that for a trans is kind of off label, but I heard it being used that way. Probably the same stuff as shudder fixx. Lubegard though has a great reputation. Many shops put it in every rebuild, so they hopefully never see a comeback. None of those products fix something seriously worn out or broken. Maybe the fluid got a bit tired or the TCC clutch needs a bit of help.

 

It's what we used when I worked at a Ford dealer.