Hilux driveline issue

[Karbunkle]

My friend has a 2001 Hilux 4WD (RN145??) which clunks when 4wd is engaged under heavy load (eg in sand) clunk sounds to be coming from front half of vehicle. Sound like slipping gear. What is the best way to diagnose the part at fault (gearbox, transfer case or front differential?)
I search but I did't find anything of use.

Thanks

[BeRad]

i think your better off asking this question on a 4x4 site....but maybe a front driveshaft/cv joint has seen better days...?. are the hubs fully engaging.....