bbforks would be correct
converting gas to lpg is fairly easy
but converting from gas/lpg to diesel is a whole different ballgame

you'll need a new engine
fuel tank (needs to be diesel compliant)
cooling system will have to be compliant with a diesel application
a battery capable of handling a diesel engine starting system (usually larger than standard batteries)

you would have to make so many changes to the truck it wouldn't be worth the cost imo.
take the lift to a traders market that trades in equipment and sell it and find a diesel lift that suits your needs and buy that in its place.