my 4x4 doesnt work after i lock it in where should i start?

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My truck will not go into 4x4. I lock the hubs shift it into 4x4 High and low but my front wheels still dont turn.What should i do where should i start looking to fix the problem. ive replaced the end of the truck 4 years ago,mew hubs,cv axels all of it, Ive never worked on the transfer case.