Sounds like you have too small of a truck or too short of a bed or the rear wheels are positioned too far forward. Can't really get specific as you don't provide enough information - forklift type, deliver truck, mounting type (manufacture's kit or homemade type). I've laoded and unload - Teledyne Princeton, Moffet & Navigators without issues & I am a sales type.

Truck mounted forklifts are commonly used here in the US and they travel with and without payloas (lumber, sod, bricks, roofing materials, etc.) at speeds well above any posted speed limit w/o issues - unless the brakes ain't too good.