Navigator transmission slips when merging onto the highway. Anyone had theirs rebuilt locally?

[oliviawilson]

I've got a 2017 Lincoln Navigator L that's been in the family since new and it's always been rock solid but the last few weeks it's started doing this thing where if I'm trying to merge onto Sheikh Mohammed Bin Zayed Road and give it a bit of gas to get up to speed the revs just climb for a second before the gear actually catches and then it clunks into the next gear kind of harshly. It doesn't do it in normal city driving or when I'm just cruising gently, only under that medium load when the transmission is trying to decide between fifth and sixth. I checked the fluid myself and it's not burnt smelling and the level seems fine, so I'm worried it might be something internal like a worn valve body or the torque converter lockup clutch starting to go. I called the dealer out of curiosity and they basically said they don't really rebuild these 6R80 units they just replace the whole transmission assembly which is a terrifying number to hear over the phone. A friend mentioned a place that does Lincoln Transmission Overhaul Sharjah and apparently they've done a few of these Navigators and Expeditions with the same gearbox and can actually open it up and replace the worn parts instead of just swapping the whole unit. Has anyone actually gone through a transmission rebuild on one of these big Lincolns and can tell me if it's worth fixing versus just nursing it along until it completely gives up? I love the truck for hauling the kids around but I'm not trying to sink more than the thing is worth into a transmission.