When I my truck shifts at WOT it sounds like the belt is squealing for a second, its pretty loud and annoying. Could it be the belt or possibly something else?

 

IF the belt is several years old you should inspect it for cracking and probably change out. Check your tensioner and make sure it is in good condition and still applying load to tighten the belt.... Sometime oil / grease gets on the belt and can make it slip. Making a solution of dish soap and water and spraying it on the belt, or dripping slowly as it runs can work around and clean up the pulleys and belts.

 

You need to replace the belt with a good grade.
Ever put some thought into why the belt is losing grip?
You have a fan clutch, an alternator, a power steering pump, an AC compressor, that all are tuning their full speed and cannot change speed that fast (inertia loading) at upshift where the crank speed suddenly is forced to slow from the transmission load.
The belt cannot keep full friction on every pulley especially if the belt gets hard and a pulley gets polished over time.
Good luck.

 

Changed out the belt with a Gates one, still chirps but just barely you have to be paying attention to hear it. What's next, the tensioner?

 

Could be.
If it's been going on for some time, the pulleys have become polished to a smoother surface and don't grip as well.
Look at the pulley surfaces for being very shinny.
Any belt driven power unit that is getting hard to turn will contribute to the issue.
Check each one by hand to see if one is beginning to go into lockup from a bad bearing.
Un necessary hard accelerations contribute to the issue and wear belts.
Good luck.