You could also check the modulating valve orifice plug. This can loosen and causes slipping mostly when hot. Although you would most likely notice this in forward and reverse. Remove the plug on the valve body that you can stick a 14mm hex bit in (side of valve body closest to torque converter). I have my bit cut to about 30mm and use a 14 mm wrench to remove plug. Then remove a spool with a snap ring pliers or something similar. On the back side of the spool is the plastic modulating orifice plug (usually red in color). Check to see that it is tight.

If the transmission was rebuilt in the last five to six years it could have some defective quad rings on the clutch pistons. The rings were made of a material that could not withstand the heat and would get hard. This could happen in a matter of a couple of thousand hours on the truck if it is run hard. Daewoo never did say they could have had incorrect material for the rings. This probably happened to at least 10 transmissions where I work.