If you have manual locking hubs and the truck moves when the hubs are unlocked then it has a part time kit in it and there is no longer a diff in the case.

Unless you are hearing some horable noises than the problem is probubly the shift linkage. They used an overly complicated box that has a bunch of springs and cams in it to make one handle move 2 links in the correct order. None of the 3 trucks I have had with 203's in them have had a non-working box. The best solution is to gut the box and put in a pair of shifters to work the linkages. You can usualy force the shifter to stay in gear by holding it forward or back.

Also check your floor clearance and make sure the lever isn't bottoming out on the floorpan.