my 99 k2500 4wd will not engage

Could be a bad actuator / encoder on the front differential You can have someone push the electronic buttons on the dash from from 4wd to 2wd and see if you here the actuator trying to engage. Sometimes its as simple as needing to clean the connectors as they get mighty dirty and oily is there is oil seepage or leakage. Also, the fluid level in the transfer case to ensure it's not low.

The front actuator is most likely bad, Put it in 4hi and see if you can turn the front driveshaft. Not to be confused with the encoder, that is the motor on the transfer case that changes the gears in the transfer case, the actuator engages the front axle. Usually when the encoder goes bad it wont change out of 2wd, still i say USUALLY! lol