If you don't already have one, you 'owe' yourself a factory service manual on this truck. It would go into detail with pictures and procedures on every serviceable part of the truck. It could become your most valuable tool when you have a problem.
Chain slack and the debris collected on the chip magnet will give you an indication of wear. Hand tools and snap ring pliers are necessary to split the case. A visual inspection of bearing races should also tell you how far you should go with it. A cleaning and fluid change at the very least.
Other manufacturers used this box, so it would also be covered in Chilton's and Motors from this era also. A good public library may even have a factory manual for loan.