I had the opposite problem.  The add on front hitch on the truck carried the bikes too low, especially when crossing ditches or creeks.  I got a Swagman Traveler XC2  two bike platform RV rack for two bikes with 2 inch receiver hitch.  I had a welding shop cut and extend the hitch tube and raise the bikes about four inches. He used two inch tubing to do that by stacking it.
Bikes
 End
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       ............                 Hitch Hole end/Bumper
Don't know if this diagram will post right but here goes.
You might be able to do the same thing by dropping it.