When it rains....it pours............

[Bishops Trucks]

I have a 1997 GMC Sierra K1500 with the 4.3 V6 and 4L60e trans. This morning I was following my wife and daughter to the local IHOP to meet some of her church going friends for breakfast. When I took off from the stop sign onto the main road, I gave it the gas peddle and all of a sudden the rpm's went way up and the trans was slipping. I had to stop twice (because it wouldn't move in any gear) and cut the truck off. Sat there for a minute and started it back up and took off. it would go for a little ways and start slipping again. When I got to IHOP, I put it in reverse to back in a spot and I had nothing. I turn it off in park, start it back up and get it into a parking spot. This is all happening in a 2 mile drive. I had to repeat the process 3 times to get it home. I had the trans rebuilt 4 years ago and have not had any issues until now. I checked the fluid cold, then started it up and checked it. Fluid level never changed. OH what could it be.....Pump?