You could buy a replacement 4.4i for fairly cheap. ~$1500ish. Seems the cheapest option. Easiest is definitely sell the truck for $1500-3000 depending on buyer. Parting it may or may not net more money and is a lot more work. Last LCI 4.4i that I parted netted just over $2000. Still had lots of parts on it but after 6 months it was time for it to go.

LoL @ "all BMWs end up in the scrap yard." A used Porsche is no more or less reliable than a used BMW. Each car's reliability will depend on how its previous owners maintained it. You've done a ton of wrenching on your X5 so I understand the frustration, but be prepared, a Cayenne could be just as problematic

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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
Nav, HiFi, 6 DVD, Sports Pckg, Cold Weather Pckg, HUD, CAS, Running Boards, Leather Dash, PDC, Pano (Purchased 03/17 w/ 136,120 miles)