Yes, I have heard that there are problems with the alternators on these trucks and depending on the situation, they may have to be replaced every year.
I have an 86 Trooper TD and when I bought the truck the alternator had just been replaced. I had the truck about six months and started noticing that the alternator light would come on randomly at idle. If I rev'ed up the motor a little, it would click and go off. This situation comes and goes but mostly comes now. I have increased the idle a little and it takes care of the problem for the most part, but every now and then the alternator light will come on as if I turned off the ignition which,in my opinion, may be part of the problem.

The way Isuzu wires up the alternator is strange: on all of the vehicles that I've ever driven, once you turn on the ignition switch, power is available to all of the systems in the vehicle. Not so with this Trooper TD: I don't have access to any auxilary power until the truck has started and the power is switched on by the alternator. Must have something to do with the revs.

Please post the alternator rebuilder's name/link for reference. $80 is a great price.

Here is an old link that I ran across over a year ago when I was looking at possibly buying one: $99 including a refundable $15 core charge.
http://autoexpress.safeshopper.com/1873/13591.htm?524

Jesse