Went 4wd low range front dif lock not a challenging hill, smoke coming from underneath, smell of clutch, just had clutch replaced 500k . No clutch slip, tried high range same issue. 

Drives fine in 2wd on issues 

Mate,

that doesn't make sense as low range increases torque ratio requiring even less power/torque from the engine which means less work for clutch!

What about in High 4WD mode or without front diff lock? Anyway I suspect your transfer case is locking up or has a fault.

Any oil in it? That would be my first check to see what oil looks like if any. Then I'd jack whole car up and drive/try to see what is going on.

Regards