Doesn't sound like silicone diff grease to me. In fact, if it looks like treacle, I'd probably bin it :-)
You want some grease like this for the main diff balls / pressure plates.
http://www.rccarshop.co.uk/index.php...diff-lube.html
And some like this, for the small thrust race: -
http://www.mk-models.co.uk/as6588-te...ase-1430-p.asp
May be useful to add pics of what you've got, and the progression of your build / project..