NP208 transfer case question

[Duro5341]

Hey guys

I have a 86 K10, with the 208 transfer case

I can switch from 2HI, to 4HI all day no problem. When I put the truck in neutra/parkl to put it in 4LO, as soon as I hit N (neutral) I hear gears grinding, and I can't go into 4lLO, or back to 4HI.......it has fresh fluid in it, and it is topped up. I end up turning the motor off, and stoping the gears from moving, to change it back

Am I doing something wrong?, or is something screwed in the TC?. I don't usually need 4LO, but I want it to work, just in case I do need it

Thanks
Cameron