96 Express - trans slipping and hard shift

My Chevy G10 Express has minor slip occasionally in low gears, but I'm more concerned about the hard shifting. This started after the local oil change shop performed a transmission flush procedure. I can make shifting smoother by quicker acceleration when taking off.

I have read threads concerning trans sensors causing slip. Could this be the problem with rough shifting as well. If so, how difficult are these to change?

Thanks.