just a little pointer incase you didn't know but you shouldn't use 4x4 whilst driving on tarmac or pathed/hard surfaces as you will get transmision wind-up.
p.s. i would go with rossco and change the gear box oil.
I would start as stated with changing the oil, new oil often works wonders as it is often overlooked on servicing.
A low oil level will also cause slippage.