203 np or 205 or 208

[speedd67]

I have a 78 gmc w/203np that I bought. Someone put manual locking hubs in and when i engage the four-wheel drive it makes really bad noises. checked hubs,u joints,and gears in front and all are fine. I believe that they changed it over to one ton cause when i needed rear break parts for e-brake that was what i had to use cause the 3/4 was different. I think it may be the chain in the transfercase. But i was wondering if i should go with the 205 part time or another 203 or change it to a 208 because that is for one ton. I need help because i don't know enough about four wheel drive to know the difference and not enough shops around to help me . Please help if some knows the difference.