I guess first is verify you have the proper amount of fluid.
Since you have reverse, I would say there is a valve sticking or not working. There should be a valve that allows pressure to the forward drum. I would check electrical connections going to the transmission. One may of been unplugged.The worse case, is to pull the pan and remove the valve body and check the valve. Be best to find a schematic of the transmission or the valve body . If this all checks out, it will be an internal failure in the transmission.
Did you just purchase this truck? Can the dealership prove with physical log or receipt that the work was done? I dont know the laws where you are at, but can the dealership be held responsible to repair this at their cost?