I can only do about 2 sets of any biceps exercise before my forearms tighten up and I can't do any more. I don't grip the weight too hard. Are my fore arms and grip just too week for the amount of weight I want to lift. My bi's don't get tired or much of a work out... what's up?

Try doing compound back and biceps exercises like pull-ups and barbell rows, You can use straps if necessary to maintain grip. Those exercises will work your biceps harder than you think.

You might turning you wrist towards your whilst doing curls, try and concentrate on keeping you arm and wrist straight, this will solve the problem I hope :P.