Do I have NP205 or NP208? Problems staying in 4x4

Hello I have a 79 Chevy with 350, SM465 and what I believe is a np208.  It is aluminum so I don't think its a 205.  How do I tell if its the 205 or 208?  The truck has warn lock-outs.  Also the shifter has been switched out to the newer style with the square plastic box with the shift position indicator window.  I was told the original shifter was just a boot in the floor.  The truck stays in 2wd properly and if I pull the shifter back into 4hi it goes in just fine.  As soon as I hit a bump and jar the truck the shifter 'settles' back into 2wd.  If I pull the shifter back to 2hi, then move it right, back past neutral into 4lo all is ok until I jar the truck again.  Soon as I hit bumps or something the shifter will settle into neutral and the truck will stop moving.  Do I have a shifter linkage problem (if so how do I asjust this) or do I have a transfer case problem?  If I go into 4wd and physically hold the shifter in gear its fine.  I would greatly appreciate any help.  Thanks