Transmission Slipping around 25mph

[ArNarX5]

Hi guys... so I have a bit of an issue and don't know which way to turn.

My wife's 03 4.4 just had over 4500 worth of work done which included pretty much everything from an engine rebuild to brakes, control arms, etc.

Prior to spending the money on the car, I had noticed a slight vibration at around 25-30mph. The car had 147k miles on it with the original transmission fluid. I bought it at 99K and the first thing I wanted to do was to change the fluid, but the mechanic said he's seen instances where people change the fluid on high mileage transmissions and they automatically fail and can't even be driven out of the garage. I read blogs and found this to be very controversial subject, so I figured I'll gamble and leave it be.

After 48k miles, the vibration kicked in and while the car was getting the engine serviced, I pulled a sample of the fluid and sent it off to blackstone labs for analysis. They came back and said the fluid was overall clean and the transmission should no really be showing any issues that would be caused by the fluid breakdown. I should have done this when I bought the car... yes, I know. So I figured let's flush and change the filter.. added Febi oem fluid.. I think about 8 or 10 quarts... new filter and the transmission immediately stopped slipping. I was happy that the gamble had paid off!


Now the car has about 152K miles on it and the slipping is slowly coming back... The mechanic said it will be another 3-5k to repair/replace the tranny.
It's a 12 year old car in near pristine condition. It would be worth about 5k max. I really don't want to spend 8K to save a car that's only worth half... but then I already put in the huge investment.

My question...
Has anyone ever had this experience of the slipping around 25mph under low load? If so, how did you address it? fix, sell, magic additive?

What would you do if you were in my place?