Rain + trifecta tends to be water in a sensor check the wheel sensor connectors and wiring.

No squirrely rear movement. Some years ago, before the lights came on , the rear brakes would apply and keep the X from freely moving forward. It didn't matter how high you idled to pull away, it wouldn't move forward quickly. Than, it would stop dragging the rear and all of the sudden snap forward and catch present speed. It only happened when first started up and pulled forward from a stop. Oddly, it reversed fine. Eventually it stopped doing it and hasn't since. BUT, for a time, going forward from a stop the front brakes would grab and shudder the car trying to move forward. It was like the brakes were applying themselves. Than, suddenly it would release and the TRIFecta would come on and the car would drive normal. That's pretty much everything I can remember regarding the trifecta. THere was the time I was driving at highway speeds and front wheels started acting like they were out of alignment. Turned out: Bad brake lines and faulty calipers. Replaced both and it never happened again. Generally speaking, except for these issues my X5 has been pretty great. It's 01 n has 247K miles. Motor n tranny still perform nicely. Handles like it should. Feel pretty lucky.