i have a 1992 4x4 300 6cyl T18 4 speed trans. i was out today and when i put the truck in 4 wheel drive i had no 4wheel drive. when the hubs are locked in it starts to pull in the front and when i start up a hill it starts to grind and i loose 4 wheel drive. it sounds like the hubs are not locking in. when i run the truck in 4 high or 4 low with the hubs unlocked there is no noise i am thinking the hubs are not locking in any ideas on what is happening

 

You need to start by removing and cleaning the hubs. They are the "weak link" in the system. I just removed and cleaned a set and found the lower aluminum housing on one of them broken. If one is broken it will pretty much negate the drive provided by the front axle. If it's not evident when you get them cleaned up, post some pictures. I'd bet strongly that the issue is with one of the locking hubs.