Fan belt squealing

[Roysbigtoys]

I have a '09 Pete 386 with an ISX 485. With the engine rpm over 1400, the fan belt squeals. It really does it with the AC on. So far I have replaced the belt tensioner and a new fan belt......they helped a little.
Once I put those parts on I started the engine and ran it over 1400 rpm for a minute, killed the engine and felt the pulleys. The crankshaft pulley is the one slipping as it was the warmest or should I say the hottest.
Do I replace the crank pulley next? Is there anything else that could be causing the belt to slip?
The truck has 519000 miles and 19000 hrs. The harmonic balancer has not been changed. I think I remembered in the old forum some talk about if the balancer was bad it would sometimes cause belt problems.
Thanks in advance.