Slight Drivetrain Jump From 2nd To 3rd, Normal?

[ace ventura]

When I'm driving at slower speeds and not going through the gears quickly I get this little drivetrain clunk sound but it's slight not jolting or anything when it shifts from 2nd to 3rd. Is this normal? Should I just change the fluid and see if that helps? I have 36k on the truck. I'm going to be towing a lot this summer so I'm a little concerned.