Personally, I would tow the truck with no worries.
In fact, I do. See sig.
However, I would have second thoughts about the Subie on a trailer.
As for the "80% rule", it seems to be a "Rule Of Thumb" thought up by some safety guru. The only place I have ever seen reference to it is on these forums. I ignore it. IMO, the manufacturer has already put a safety factor in his numbers. While I don't know what that safety factor is, I am sure the Mfg. kept it in mind when assigning the ratings. There is always a huge difference between "working load" and "failure load"!
NOTE: I did NOT say you should go ahead and tow your truck. I simply said I tow mine, no more, no less!