4x4 not engaging

I can’t get my '99 Ford F-150 to shift into 4wd. It has a floor lever which moves but with difficulty. Nothing lights up on the dash. I’ve actually had problems with this since it was new but never got a satisfactory answer from the Ford dealer.

Is there a way to determine whether the problem is in the linkage or switches? Can I get under the truck and try to move something?

First you must determine if moving the lever to the 4WD position mechanically engages the front axle.Jack up the front wheels and get under the truck and with the truck in 4WD the front drive shaft should not move but when shifted to 2WD the the drive shaft should turn and the front axles should engage to turn the front wheels.