What size allen wrench is needed for the drain or filler plug on 98 blazer transmission?

Do you mean differential? My '98 Blazer transmission doesn't have a drain plug, and I fill it through the dipstick tube.

i dont know what size it is, at least right now but i know that you can defitnitly get that big of size at sear when i needed one for my jimmy it sears was the only place i could find it at, good luck

Screw it, I think I will just siphon it (yummy!) out through the dipstick tube rather than buy a tool.