Rear Tires Suddenly Lock Up Going 65

I havn't touched the brakes for a while. I used the brakes leading up to the on ramp but while accelerating, everything seemed fine, til I hit about 60 or so and the "lack of power" came about. I do use the E-brake a lot when I park in the driveway. But today, the e-brake wasn't used.

I don't recall engine speed. It all happened quick, at that speed I'm at about 3000 rpm and I think the engine speed went to idle as soon as I let off but I'm not sure.

Oh and I did change the diff fluid and added a friction modifier for the suregrip when I changed the tranny fluid.