4x4 not working (grinding) help!!


I have a 1990 2.8td fourtrak with manual hubs. I went on a field to test four wheel drive and when i select 4x4 and turned the hubs to lock, i drove off and no problems. only when i can to a sticky patch did i realise that when the front wheels try to power there is a grinding noise.

could any one help with this?
the noise sounds like the gear slipping or something