D1 slipping under load

I'm hoping you can help me with some diagnosis. 97 D1 automatic.

Over the weekend, I was on a trail climbing a steep hill. I heard a mechanical 'whizz' and started getting this funny feeling in the driveline. Kind of like a slipping clutch (just trying to describe it - my truck is automatic) that felt like it was from the rear wheels. Almost like driving in light snow where you feel the engine, but the wheels don't hook up for a slip second. I made it up the hill and decided it might be the abs (light has been on for a while and the pedal has been acting up lately). I pulled the fuse and kept on going. We we're done with most of the climbing. I couldn't really recreate it and figured it was in my head. Drove home fine.

Today, I was pulling a trailer from the bottom of my yard and it happened again. The trailer is full of construction trash and weighs more than 1k pounds by now. I had it locked (steep yard) and under full load/torque, I felt a little slip and heard a little whizz. I unlocked and pushed the gas down and couldn't recreate it (by then I was on the driveway).

Any idea what could be going on?

I thought abs was involved because I had been having more pedal feedback in the last few weeks. It was also the only thing I could think to try. You are correct that there is no etc system on my D1.

I didn't hear the wheels slipping.

I'm pretty new to off-road driving, but I've had the truck out more than a dozen times on Colorado trails and have put about 10k miles on it. I'd like to believe I'm familiar with the feel of the truck while climbing. It's hard to describe, but it feels different/off.

Along with a big maintenance push, I'm doing the diff, transfer case and transmission fluids. Also having friend rebuild the drive shafts. Maybe something will jump out

I did the transmission filter around 170k. No crazy smells or metal flakes. It has about 175k on it now.

The diffs were dirtier than I expected for only having 10k on the fluid. Not sure how frequently they were serviced before I got it.