Insulating ducts in a crawlspace
Any time you have heating ductwork in an unheated area, it is very wise to insulate it. There are several products, but I tend to go with the high density, foil backed fiberglass wrap most of the time. The exact type differs depending on the shape of the ductwork. (round vs rectangular) I always seal any seams in the insulation with foil tape.  To solve the uneven heating problem, you will need dampers in the shorter runs. By adjusting the dampers, you can balance the amount of hot air delivered to each register.