Full locking rear differentials

[DVT873]

Volvo, you might look into Automatic Traction Control (ATC) for your ABS. It works fairly well. I had a customer who ordered some single-axle straight trucks for job site work. He disengages it when he goes onto the site. If he starts to get stuck, then he engages it and tries to back away from where he is. He says it works well for him. I realize this is not quite the same as your condition.
You say this is only a few times a year, and you usually can get pulled out. Maybe leave it at that. The full locker setup is going to cost a lot of money, reduce mileage, and wear out quicker. Keep in mind that if you get that combination stuck, you will be really stuck. All 4 wheels spinning and digging holes. And if just one finds traction at the wrong moment, bye-bye rear end.
A long time ago Pennzoil ran some barrel trucks near me. Tandems. They ran normal power dividers but had Detroit Lockers in the rear rear diff. I got to know the mechanic there. Sharp guy. He said that combination worked out pretty well for them and that was all he wanted. They had tried some full lock-up setups. Drivers would forget to disengage.
—an expensive mistake. This was before electronic controls. And some still got stuck.
So they settled on the Detroit Locker in the rear. Fairly simple to add IIRC. It replaces the existing spider nest. Downside is they can be noisy.