Outer Limits 4x4 Board

Hey people, I just bought a HJ60 landcruiser about 5 months ago and it pops out of first sometimes.
If you rest your hand on it and take off before removing your hand, you can drive in first all day and it wont pop out but if you put it in first while stopped and take off without your hand on the stick it will pop out.

Any ideas on how to fix this problem??

Dont know much about 4X4's at all so any info will be appreciated especially if I can understand it.....lol

could be 1 of 3 things the dog teeth have worn and it will just pop out meaning full gbox strip and rebuild
a broken detent spring located in the top hat ,need to remove the floor cover then you will locate the detent keeper bolts remove one at a time and check the free length of the spring against toyota specs
the nut has come loose on the end of the main shaft ,remove the rear bearing tin plate and tighten the nut to specs use a new nut if possble
if the 2nd 2 repairs dont work then resort to repair 1

Thanks mate not sure what any of that stuff is that you mentioned but I will do what you have said and hopefully I will be able to identify what you are talking about.

Thanks alot for your help