What kind of vibration and how much? As engine torque is applied to the drivetrain, especially when first pulling away from a stop with a trailer, some vibration should be expected. Also, if the vehicle is equipped with a manual clutch as opposed to an automatic transmission it may cause even more vibration.
[S. Nix] Assuming this is a vibration that you feel, have the drivetrain checked. Especially the universal joints and center support bushing. These can give vibration on acceleration especially with the added load of a trailer. Another less likely cause is an internal transmission problem.