Have you for sure ruled out ANY possible vacuum leaks in the turbo boost controls or a problem with the turbo vacuum solenoid valve or modulator valve.
The small plastic tubing going from the turbo modulator valve over to the turbo vane actuator are notorious for having a small crack in them or partly collapsing due to heat. Need to check ALL vacuum hoses and tubing very closely as any leaks or partial stoppages will greatly limit turbo boost performance.
P0299 -- Turbo/super charger – low boost Mechanical faultNext possibility; with the engine off, check the stroke of the turbo mechanical vanes by manually operating the actuator with the vacuum line disconnected.
Several owners have reported in the past of finding their variable vanes froze up or partially froze up. If this condition is found, you can try spraying the linkages and vanes from the outside of the turbo with a good penetrating oil like PB Blaster or Kroil and manually stroking the vanes to obtain full travel. But in most cases it does require the removal of the turbocharger and opening it up and physically cleaning the vane operator mechanism. (Item #5 in the picture below)
See this vane cleaning trick that some have used using oven cleaner to keep from having to remove the turbo.
VNT turbo stuck vane cleaning FYI
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Supporting Vendor and Moderator of LOST05 Jeep Liberty CRD Limited
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