....yes the diffs are open. But that doesn't change the fact that the front and rear tires on each side of the vehicle need to move at different RPM in the event of a turn and are powered at the same RPM by the engine.
Feel free to correct me, but I believe open diffs alleviate the side-to-side issue, but do nothing to correct the front-to-back problem in the event of part time 4wd. You need a center differential for that.
I don't disagree with following Toyota's recommendations on maintenance - but if you're going to do it, you're best doing it on the proper surfaces. I would say performing this "preventative" maintenance on dry pavement is probably a wash.
Toyota states their monthly recommendation is to ensure lubing of all the 4wd equipment. The only thing that's not really spinning in 2hi is the transfer case...the rear diff is powered and the front diff is spinning along...so they're both in the oil...
Edit: And those ten stupid things are all good advice. You should keep readin.