it can be any one of a number of things,i would start with the tranny fluid and filter change.when your truck is good and warmed up,check the fluid,not just to see if its low but the color,and the smell off trans fluid can tell you alot,if the color is dark like blood and it smells burnt,then you may have real problems.i would start with the fluid,if that doesnt help take it to a trans shop for a free inspection,let them figure it out and then you take it home and fix it.good luck!!
BIGBLOCK MAN