V3500 funny noise, funny situation

[MTCK]

I dug out my 91 V3500 to help a coworker who's Ferd died during our current cold snap. It's a F350 Powersmoke flat bed, ~9,000 lbs empty and he needed a tow up to his house. No problem, except the house is at the top of a ~12% grade, 300-400 yards that is covered in hard-pack snow.

First attempt I was in 4 hi, 1st gear and was powering out and loosing steam until it started to spin, then it was all over. As I started to loose steam, (WOT) I started to hear a loud click (soft bang?) with increasing frequency as I slowed down. When progress stopped, I got out and it was evident from the tracks that I was spinning one tire in the front, and one in the back as you might expect with open diffs.

Second attempt, I had chains on the rear, was in 4 low, 1st gear. I had plenty of power this time, but again started to spin about 30 yards further up the hill and then lost forward progress. Same noise as before, seemed to be same frequency. Didn't try a third attempt, he needs a grader or a tow truck to get that pig up the hill.

Drivetrain is as shown in my signature. The only slop I noticed when doing the once over when I bought the truck was in the rear driveline / t-case. With the shifter in park, I could turn the rear driveshaft 5-10 degrees and it would hang and click slightly when it released. I thought it was odd, but plan on going through the drivetrain thoroughly when I repower the truck so didn't lose any sleep.

Any ideas? It drives smooth as the day is short (in AK in the end of November) around town in 4 - hi. The long wheelbase and tall skinny tires are great on slick roads compared to my SWB!

Thanks.