My 88 Chevy silverados 4x4 wont engage in place?

When driveing my 88 chevy silverado 4x4, i go offroad alot. But once i get stuck in 2WD, i go put it in 4WDL, (4WDH doesnt seem to work). But just puting it in 4WD doesnt make it go into 4WD. I have to move the truck forward, or backword a few feet and hear/feel the pop and then it is engaged. Is it supposed to be like that?