Oh my god I fixed it myself
Absolutely ZERO idea why this handling line works, but it's the one I used for the Tahoe and Expedition. The original one I used that caused the reversed suspension behavior was the Gresley-based one.
Handling line I used:
<Item type="CHandlingData">
<handlingName>14FPIU</handlingName>
<fMass value="2130.024000" />
<fInitialDragCoeff value="1.400000" />
<fPercentSubmerged value="85.000000" />
<vecCentreOfMassOffset x="0.000000" y="0.065000" z="0.050000" />
<vecInertiaMultiplier x="1.000000" y="1.090000" z="1.210000" />
<fDriveBiasFront value="0.270000" />
<nInitialDriveGears value="6" />
<fInitialDriveForce value="0.188000" />
<fDriveInertia value="0.800000" />
<fClutchChangeRateScaleUpShift value="2.900000" />
<fClutchChangeRateScaleDownShift value="3.300000" />
<fInitialDriveMaxFlatVel value="140.000000" />
<fBrakeForce value="0.250000" />
<fBrakeBiasFront value="0.602800" />
<fHandBrakeForce value="0.600000" />
<fSteeringLock value="37.000000" />
<fTractionCurveMax value="1.450000" />
<fTractionCurveMin value="1.240000" />
<fTractionCurveLateral value="16.500000" />
<fTractionSpringDeltaMax value="0.130000" />
<fLowSpeedTractionLossMult value="0.400000" />
<fCamberStiffnesss value="0.000000" />
<fTractionBiasFront value="0.483900" />
<fTractionLossMult value="0.870000" />
<fSuspensionForce value="2.000000" />
<fSuspensionCompDamp value="1.650000" />
<fSuspensionReboundDamp value="0.400000" />
<fSuspensionUpperLimit value="0.140000" />
<fSuspensionLowerLimit value="-0.122000" />
<fSuspensionRaise value="0.000000" />
<fSuspensionBiasFront value="0.530000" />
<fAntiRollBarForce value="0.090000" />
<fAntiRollBarBiasFront value="0.530000" />
<fRollCentreHeightFront value="0.210000" />
<fRollCentreHeightRear value="0.200000" />
<fCollisionDamageMult value="0.000000" />
<fWeaponDamageMult value="0.000000" />
<fDeformationDamageMult value="1.700000" />
<fEngineDamageMult value="0.000000" />
<fPetrolTankVolume value="65.000000" />
<fOilVolume value="5.000000" />
<fSeatOffsetDistX value="0.100000" />
<fSeatOffsetDistY value="0.000000" />
<fSeatOffsetDistZ value="0.000000" />
<nMonetaryValue value="50000" />
<strModelFlags>440010</strModelFlags>
<strHandlingFlags>820100</strHandlingFlags>
<strDamageFlags>0</strDamageFlags>
<AIHandling>AVERAGE</AIHandling>
<SubHandlingData>
<Item type="CCarHandlingData">
<fBackEndPopUpCarImpulseMult value="0.100000" />
<fBackEndPopUpBuildingImpulseMult value="0.030000" />
<fBackEndPopUpMaxDeltaSpeed value="0.600000" />
</Item>
<Item type="NULL" />
<Item type="NULL" />
</SubHandlingData>
</Item>