Truck lurches and knocks?

I have a 1991 Ford Bronco XLT 5.8L V8 with an automatic transmission. When I accelerate from a stop the truck lurches forward as if the gears are slipping in the transmission. It doesn't do this every time and when it doesn't it makes a knocking sound once I hit about 20 mph. Any ideas?

Change the tranny oil and filter. Make sure to put the correct amount. Do not overfill. Collect the oil and put the same amount of oil that came out.