Have you done your differential fluids? Front/Rear? Not sure it fits your symptoms, but that front diff sees a lot of heat and the fluid gets really nasty.

As far as the trans. If it is a 6HP. You could attempt a service but just a drain and fill. Do not machine flush. If you are sure it's the trans causing your issue, I'd do the 4 tube seals, the bridge seal, and then fill it up with Valvoline Maxlife ATF (has to be multivehicle). Be sure to follow the fill procedure.

^ But that's only if you have exhausted other possibilities. If the trans dies (mine did when I threw a hail mary at it after the E clutch bushing failed), paying someone is very costly, if anyone even wants to touch it. Shops act as if ZF transmissions are magic black boxes. I did it myself to replace on my wife's diesel and it still cost near $1200 in parts. More if you include the "while I'm in there" parts list.

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2005 X5 4.4i Build 04/05 Maintenance/Build Log
Nav, Pano, Sport (Purchased 06/14 w/ 109,000 miles) (Sold 8/15 w/121,000 miles)

2006 X5 4.8is Build 11/05 Maintenance/Build Log
Nav, DSP, Pano, Running Boards, OEM Tow Hitch, Cold Weather Pckg (Purchased 08/15 w/ 90,500 miles)

2010 X5 35d Build 02/10
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