I don't know how common this is with GX470s, but it is somewhat common with 4th gen 4Runners. My GX was very hard to steer, almost like manual steering. To cut to the chase, the U-Joint in the lower steering shaft can seize. I had flushed my PS fluid and almost replaced my whole steering rack before I started cross-researching with 4Runners. The shaft in the 4Runner is slightly different, but it is a very similar solution (actually, now that I re-look at the 4Runner thread, the part looks damn near identical): 4Runner Steering shaft replacement

Because it is hard to see the lower U-Joint, you may not notice how bad it can rust. Mine was almost totally seized in one direction. It seemed to come on quite suddenly, although my wife drives the car more than me. Going through the DS front wheel well, as in the thread linked above, is the way to go. Mine didn't look terrible, but there was quite a bit of hammering involved to slide the collars off their respective shafts. It is only 3 bolts, but a lot of work freeing the shaft from the couplers.

Be careful with lining up the shafts, my steering wheel is now off about 1/8 turn, but it needs an alignment anyway so hopefully they can adjust it out.

 

Just noticed some tight spots in my turn and went ahead and flushed the PS fluid to some relief but I’ll be checking this out next. Thanks!

 

I had exact issue. No luck of changing power steering plump. While doing search, I found the U-joint seize video.