what's the destination app? try working from the original .obj rather than an export. There's a lot of variables that can only be addressed once the destination app is known. For instance, if I import the v4 original .obj (not an export) into ZBrush from the geometries folder the groups will be intact but the mesh will not be unwelded. If on the other hand I import the same .obj into Wings, the mesh will appear unwelded and broken into body part groups (and nonsensical ones, at that). In Modo the mesh is of one piece...

Also, if morphing is your goal, don't weld the figure upon export from poser. There's a good likelyhood that this will change the vertex order. see this thread for some spot-on information and a UVMapper technique for re-establishing correct grouping on an ungrouped mesh_

http://www.renderosity.com/mod/forumpro/showthread.php?thread_id=2869544

Another for instance is the import settings and group handling of the destination app, if any. Carrara for example is very Poser friendly in this regard...