AT/Auto Hubs 4wd/4hi Top Speed

I don't have the owner's manual for my '88 LE auto with auto locking hubs and electronic switch/push button. What's recommended top speed for the AT with auto hubs in 4hi? It doesn't say anything about a top 4hi speed in my '88 Pickup manual but hopefully there's a note in the 4wd Van manual if someone has it.

A couple of our hwys here have mixed conditions where I'd much rather leave the van in 4wd than pull over and change it in/out all the time. I drive 60ish mph typically where it's straight and safe, more like 40 on the curves. I like the 4wd on as it keeps the rear end under control on ice and snow. Running fresh general grabbers which are great for snow but aren't actual winters that grab ice so the 4hi helps a good bit. I just don't want to push the t case too hard.

Thanks!