I was out using my cherokee and I put it in 4h and 4L when I was trying to get up this hill on loos ground. My back tired kept spinning but the front one didn't move so I'm under the impression that my 4x4 system doesn't work. It's a 1990 jeep cherokee pioneer. Any advice would be great

 

search 'front axle disconnect'

theres a vacuum-operated piece in your front axle that disengages the front wheels. there are a few ways to fix this from cheap DIY to sort of expensive bolt on kits.