any insights into where the best places to watch for a used tow vehicle < $10K may be? i'm watching craigslist, ebay, etc. but I am not finding much. Is there a government auction site or something that may be helpful?

Government Liquidation, Iron Planet?
I usually look in the desert states, no rust. I have had both the newer-shorter/narrower and the older-wider/longer (1988/9). I would switch back to the older heaver in a heartbeat.
My first choice would be a Ford Excursion with the older Diesel. Good luck and I really relate to your avatar.

Glad you like the avatar! it's all about contingency planning

I can't say for certain that the USG Fleet vehicles were 8.1L trucks. I know they had a LOT of 3/4-ton Suburbans, but I bet most were 6.0L.

Make sure you look for 2500 Avalanches too. ALL of the 2500 Avalanches have 8.1L / 4speed auto transmissions. We had two of them in my family.

I currently have an 8.1L/Allison 2003 2500HD Silverado LT. It's pretty much my DD and local/light towing/hauling beast.
While I ABSOLUTELY LOVE the truck, I am re-evaluating keeping it long term. I just had the radiator crack on me this weekend while away from home (towing); and it was not easy locating a radiator to fit it. It has non-standard connections (quick-connects, not barbs). There are already parts that are NLA for these trucks (Namely the Cam Position Sensor for early models. Fortunately, I have a spare.)

I have an 8.1L/Allison GM and a 7.3L PSD Ford; two trucks I really love and am proud to own. But it's getting to be time to reconsider getting a single truck & something newer.

Don't for get qt of oil every 1000 miles in the 8.1- I have owned 2. Strong motors for sure.
still have one in a 02- 3500 chevy van

Having driven government vehicles for 16 years, I can only cringe and pity the poor souls that buy them. Public safety vehicles may be taken out of service at the 80-100k mileage mark, but most have an additional ton of hours idling. They don't ever get premium gas, and they get whatever bulk oil the .gov contract permits, with service intervals the .gov or fleet maintenance contractor allows. And that's with assigned vehicles. Get a pool car and who knows when it was actually serviced.

gov deals bought a few things from them there ex military police stuff like that