I am hoping someone may input on a problem I've been dealing with for over 15 years.
I have a 600' long driveway, 10 ft wide that goes straight uphill a pretty steep grade. The driveway is 6 inches thick and fiberglass reinforced (BAD IDEA IMO now) but it was the contractors choice at the time. It was poured over a gravel base that was well-established for about 10-15 years).
About a year after it was poured it appeared the center joint (cut in) was opening up and the slabs were sliding sideways. The contractor came out, was puzzled saying he had only encountered this once before. He proceeded to drill 6" holes down the side he thought was moving (which is open to the entire front acreage). He then filled the holes with concrete. That seemed to slow things down but not stop it.
On the other side of the drive, is a 12 foot wide V shaped drainage are about 15-20ft wide for the runoff from above with a grove of trees another 20 or so feet wide. I even reshaped the drainage area as there was a slight high spot that I thought may be sending water under the drive during the spring thaw.

I then wondered if it was dropping on the outside edges due to the weight of delivery trucks, tow trucks etc. But the edges of the joint are not angled (V's) as one would expect if this were the case, but straight up and down

So, I am wondering if anyone has encountered a problem like this and or what might be done to stabilize it. It's not sliding down, but rather sideways. Originally, the problem was most prominent in the mid-way up the drive, but has slowly moved both directions (up and down) over time. The driveway otherwise is in perfect condition with not heaving, cracking etc.

It is an annual thing to re-caulk a substantial portion of the center joint and I have likely spent more $$ keeping it sealed than I did having it done (12-15K?).
Any experiences, thoughts, suggestions, comments appreciated