transfer case
 

Hey i have a 1990 chevy full size blazer when i put it into 4WH OR 4WL it dont lock the front wheels in gear im not sure why can someone help me think of somthing

If you have the auto-hubs that many came factory with, they likely are gummed up inside. You would either need disassemble, clean, and reassemble them or just remove them and replace them with manual hubs (recommended).

i too recommend the manual hubs, the ones with the little dials where you actually have to get out to engage them, having owned k5 blazers with auto hubs and have friends who had them they are prone to failure, nothing like seeing your axle slam in and out of engagement when you are offroad. 12 years of using manual hubs the only failed on me once, thanks to some deep water diving then freezing the next day