There was a thread about a month ago, or was about something else and ended up talking about this very subject of locking the center diff.... Anyway, long story short, I had a 1/8 scale truggy and was breaking front drive shafts way more than I liked. I thought I would turn the 4wd into a 2wd... one of the worst things I ever did to a RC. You will be able to do spins and slides pretty much on command.
I'm gonna just say to use JB Weld or something like it to lock the center diff. I don't recommend it. It'll will be pretty much permanent. I've used poster putty,
silly putty...etc. that stuff all ends up the same way... a big pile of warm, sticky mess.