You can buy just the pulley and replace it (along with the belt of course). I had the same pulley seize. I got mine from the local Checkers Auto and it was under $20. Definately replace the belt though...the friction of the belt sliding on the non turning pulley causes a lot of heat which weakens the belt.

As far as mud...be carefull with your alternator (night want to buy and carry an extra). The alt. sits very low in the VX. Clear water does not seem to affect them much. Mud and gritty waters however have claimed the death of many VX and Isuzu alternators!