Dying Blower Motor/Electrical System Can't Keep Up with AC

[fledermaus]

Lurked for awhile but this is my first post, so if this is covered somewhere I haven't found yet, feel free to point me in the right direction.

I have a 94 C1500 Sierra with the 5.7l. I don't drive it much (it's got 133k miles on it -- I've put 6k miles on it over the past 2 years). We took it out about 2 weeks ago to scout some off-road trails for a race. Ambient temp was about 90*. We were driving slow (lots of stops and starts) with the AC on, blower on high. I noticed the gauges were getting down around 12 volts. Eventually it stalled while I was driving on one of the paved roads (about 45mph), so I killed the AC/blower and restarted the truck. We ran for a while with no AC and it seemed to charge back up. This only seems to be a problem when we're not driving at a sustained speed.

AC was recharged last year and is cold. Alternator replaced in 2009. At idle with nothing on, reading 13.9v at the battery; when AC kicks on and fan on high, it's around 13.6v. (Gauge voltmeter reads ~ 0.5 v lower than what's at the batt).

Some culprits -- I'm wondering where the biggest problem is or if the rest are just red herrings.

* Blower motor squeals on Med (the 2nd position). It has no Low. I got it this way from the PO and replaced the resistor. It immediately blew the low position. Wondering if (in its death) it is drawing a lot of amperage and just slowly draining when we're at lower RPMs (like under 2000).
* Idler pulley bounces when the AC comes on, otherwise it stays stable. Appears to move approx 1/2" under load (no real squeal -- wondering if the spring actuator is tired -- or is this normal?) If this is more than normal, perhaps the alternator is slipping under load as well?
* Red Top Optima was purchased in 7/2007. I've had mixed results with Optimas, but I think most of that had to do with their use (no battery tender; used on vehicles that drew a lot and sat for long periods of time).

Where should I start? I was going to replace the blower motor and yet another resistor. I am wary of blower motor work as the daily drivers I've done in the past seem to have had the blower motor hung on the assembly line, then they built the car around it. Looking at the Haynes for the truck, this seems like a super simple job. Is it really as easy as it looks?

How about that idler pulley?

And as far as parts suppliers go -- I had my first interaction with LMC a few months ago. They did a great job, but I was a little surprised at the aftermarket parts. I'm used to going aftermarket with my Swedish cars because genuine parts are so darn expensive, but given the lower costs of GM parts, should I just go Delco?

Thanks.