Re greasing cv joints

[dazandcaz]

For a little while now i have had a knocking noise when in 4wd and the steering is anywhere other than central, strangely it goes away after a few mins of being in 4wd. I presume it's the cv's and want to try greasing before replacing. Can i just slide the cv boot back and apply grease or do i have to take joints out to do them? Would moly grease be suitable.
Thanks
Dazza