I'm with Jack ... I'd take it in for service. You own a TRUCK, and it's made to haul stuff!
If you're not able to pull a trailer that is well within the limits of what Ford says it can tow, it needs to be fixed. Plain and simple.
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2008 F150 Supercrew Lariat 4x4
5.4L - 4.88 Gears
PHP Gryphon Programmer - BDS 6" Lift - Magnaflow Dual Exhaust - JLT Ram Air Intake - 20" Pacer Ridgeline Rims with 35" Mickey Thompson Baja ATZs Tires - Westin BullBar with Driving Lights