Are you sure it's not the excess grease being flung off of the u-joints after lubing them? I know I have the flung grease ring on my truck, and after being coated with dirt, etc. you can't tell the difference by looking. Stick yer finger in it, feel it, smell it (this is not a reccommended procedure for finding out about stuff in general <img src="/forums/images/graemlins/drunk.gif" alt="" />, but I think it safely applies here).

1997 T 100 4X4 ExtraCab
5 speed, White
Worthless factory bedliner
265/75/16 Firestone Destination A/Ts
Aisin Manual Hubs
90k miles