rear end steps when accelerating and decelerating

Started this morning on my way to work. I replaced my brake matter cylinder last night, first time I've had real brakes in forever. About half way to work I let off the gas and it felt like the back end broke loose. Then when I accelerated again it felt like it kicked back in. the rest of the ride to work it did the same thing. Anyone experienced this?

I'm not sure what you mean - do you mean that the front brakes grabbed or locked up? I had that happen once after swapping master cylinders (I had to adjust the pushrod length a little shorter).

Sure it's the brakes? That description sounds like a CV joint.

No, I'm not sure it's the brakes, I just mentioned them because I did the work recently. At first this morning I was considering it being one that might have been catching but on the way home from work I ruled that out. It doesn't change with braking only when I accelerate or decelerate. It feels like you're in an old V8 car with worn motor mounts but coming from the rear. I pulled my bed off and checked to make sure my lowering blocks were still in place and no signs of a broken leaf either. I went ahead and backed them off and tightened them back down since I originally did them one side at a time. I read somewhere that could be a possible issue. I didn't check the u joint, but I will tomorrow.

Sounds like a U joint to me. Any vibration while driving? Does it change with speed or load?

When my U-joint was toast, I felt a slight on-off throttle stepping out. Fancied or real, but that's what it felt like. Check your U-joints.