Here was my "symptom" of bad auto hubs in my '83, with some background.
Had just recently taken them apart, cleaned them up, re-greased, and checked for proper operation.
Took the thing up to the mountains, approached a stream crossing that has a steep, typically slick exit, but nothing that will stop a vehicle with 4wd and good tires. Stopped, put in 4lo (to keep from destroying the exit) with plenty of room to the crossing, and started across. As soon as the rear wheels hit the hill, it was obvious the front wasn't locked. I could hear the "whirring" noise, but wasn't sure what is was. I tried a couple more times, no dice, still no 4wd. So with this being my only way out, I hit the exit running, and got about halfway up before I lost momentum, and the rears started spinning. I just kept it pretty much floored, and finally *BAM* the front hubs locked and I got up the hill. I'm really surprised I didn't break anything when they locked, but the tires were kinda bald and only 31's.
So anyways, if you've had occassion to be in a situation where you needed 4wd, and noticed it wasn't working, but you still had forward movement, it is likely the POS auto hubs.
If you like "stock" stuff, usually the 1/2 ton GM spicer hubs can be purchased at wrecking yards or from individuals parting trucks, typically for around $40. Have a pair of old Spicers on my truck, about 3 years on them, haven't let me down yet.
