Have you ever had a riding mower in pickup with ramps?

Yes, metal ramps for this exact purpose are readily available. Your local mower service centre will have some for collecting/delivering mowers, they may not mind you asking where you can buy them from. If you are leaving the cutting deck on (and assuming it is centre-mounted) the key thing is to ensure that the deck does not snag as the front wheels go down on to the bed of the truck, the ramps need to be long enough (or arched) to give a fairly shallow gradient as the tractor goes on to the truck (and obviously the deck must be in highest position). As already said, the ramps must be fixed securely to the truck. Do not use timber, there is a risk of splitting/breakage, which could result in a serious and expensive accident.