ABS is not armed , or is in standby until passing 10 mph or more.
The posi lock, which is probably just like the my 93 has. Is just that. A manual engagement device. If you don't use it or engage front axle, and select low range, then you just have a 2 wheel drive low range, there is no conflict. Adding the posi lock actually gave you another low range option. When you select low range the ABS is now selected out of action. No ABS in low range by design. Are you sure the front wheels were locked? Could it have been during brake application while launching the boat , on a slope , light weight front end, all heavy weight at rear, tongue weight, rear motion inertia, that the front wheel braking was just more effective than rear axle brakes? Which appears that there is front brake problem but is normal laws of physics. Weight, mass , inertia, deceleration forces. It happens alot during boat launching at the ramp. You have some information now. But trying to diagnose something far away . We could ask more technical questions but I believe that would be beyond your ability experience level. And yes we all started at zero.
If do not have the OE service manuals, since your gonna be in the great frozen butt north, download the free manuals for your year, search at top " service manuals " get the link, or maybe someone can post the link, please. The GM manuals, get both, Vol 1, or book 1. And Vol 2, or book 2. You need both. Everything is in there, it's your new best friend, sometimes you need to page and read through.
Why did your nephew select low range? What purpose?
Good luck, tell Elmendorf I'm never coming back.
Thanks for the reply. I understand how the Posi lock works. My boat is 7000 pounds and if the tide is not at its highest point the ramp is always slippery so the front wheels almost always lock and skid until you step hard on the brakes and fully engage the rear drums. As I thought through it more the fronts could lock on snow or ice with the rear still turning so this probably would not cause a code so it must have just been a coincidence. I am certain the light came on while launching the boat that day. According to my mechanic that 10 mph noise I heard since I bought it 10 years ago was an ABS self test. It has stopped since the ABS light came on.
After reading other threads here I am going to look at the following. In no particular order
1. Check tire pressure and rotate tires
2. Check driveshaft and U joints
3. Inspect all front end parts again (I put on a new Red Head box last year)
4. Read ABS codes - Please suggest a code reader that will bleed the ABS unit
5. Remove drums and inspect all hardware and replace drums, shoes and springs