my 2004 chevy silverado 2500 hd wont engage front wheels in 4hi or 4lo

Have the accuator checked on front axle, they are common problem on gm 4wheel drive, they are not to expensive, do not go to salvage yard, you might buy one that is as bad as what you have, they only take about 15 minutes to change.

Changed it not the problem

Do you have a floor shifter or dash mounted switch,if dash switch,possible, or if floor shifter check switch on transfer,case connected to or close to shifter, also check all fuses.

dash switch and checked everything else

Being that you have dash switch, they are a problem , also if you have hoist at your use, put truck on and try to see if you can put it in four wheel drive while running and see if front driveshaft is turning, if so then you have front axle problem, if not then problem is in transfer case.

if it is in the transfer case what is the problem with in the transfer case?