1999 Chevy Venture

I have a 1999 chevy venture mini van. It has 90,900 miles on it. It needs a new belt idler pulley, which I have been putting off. The squeeling has become loud enough to annoy me, so I will be replacing the belt and idler next week. However, starting last week, the transmission has started to shift hard when accelerating.(I have checked the fluid, the level is fine, it's brownish-red and has no odor. Should the filter be replaced?) It does not do it all the time, only after driving for more than 20 minutes. Then, today.I noticed a slight whining sound. Does this mean my tansmission is shot or on it's way? Advice appreciated.