Helping Out a Friend

So this happend before to my friend's 1999 Blazer. What happens is the car accelerates fine but at WOT it barely accelerates and it feels like the transmission is trying to hold the motor back. When you do go WOT it even sounds like the transmission is trying to hold the motor back. But like I said, under regular throttle it accelerates and shifts fine. Do you think it is something broken or something electrical like a TCU? I was almost thinking parts may be broken in the transmission and start clogging up the trans filter under harder acceleration. Any other thoughts?

Also, there could be something wrong with the valve body. When was the last time he changed the fluid and filter?