@jpjp. My transmission has been throwing fault code P0783-07 (3-4 SHIFT MECHANICAL FAILURE) for some time now and it's been getting worse. It only happens between 3-4 gear shifts when under load and/or when driving the car for extended periods of time. It will first give signs of delayed shifting into 4th (rpms slip upwards of 3.5k rpm under normal driving) and then start to slip completely out of third when driving for over an hour.
The same behavior happens when driving uphill (I live in Bay Area and there are plenty of these around) or when needing to accelerate onto the highway. The car will throw a fault and then go into limp mode. I then have to pull over onto the shoulder, turn off the car, turn it back on and continue until the problem happens again. Which could be several times during the course of the same trip or sporadically throughout the week.
It's happened enough times to cause concern where I don't feel comfortable letting my wife drive around with others in the car (namely our 3yr old).
I took it to the dealer, they recommended replacement for $8.5k! I later took it to an indy for a second opinion and they recommended a drain and fill of the trans fluid as a first step to see if this would fix. I went with the drain and fill. It improved slightly after a 2 weeks and then started back at its old tricks and progressively got worse.
Informed the indy and he said that he too recommended a replacement at this time. His quote was also a little over $8k.
Called the dealer back and he told me of a ZF transmission specialist that he's been personally going to for the better part of a decade who would rebuild mine for $3k less that what he quoted. Went with the rebuild. Cost was $5,400 for the trans. Dealer did the R&R and trans shop did the rebuild.
Coincidentally, my truck also needed the timing chains done (now back on on topic

). Confirmed by both indy and dealer. I had the dealer perform this service as well after my trans was rebuilt. They also replaced the water pump, belts, and spark plugs (no labor charge).
The dealer has been very helpful through this whole process. Not to mention they loaned me a new 5.0 SuperCharged RR to drive around for 3 weeks while the repairs were being made.
I pick up the truck tomorrow and will report back on all the repairs. The shop even sent some photos and videos with the engine tore down to show the ear on the chain. I'll have to post when I get the chance.