Hey everyone, posting here so my experience may help others.
I have a 2006 4.7 with 300K miles. Recently, I started to get a slight hesitation under throttle--typically 2nd and 3rd gears. It didn't happen all the time, idled perfectly, and didn't throw any codes. If on the highway and I give it the beans, it would accelerate without issue....only at at lower gears would I feel the hesitation.
I checked the fuel pressure and it was reading 35psi, but would drop to about 25 for a second when I snap the throttle and then return to 35. I think the spec for pressure is 38-44psi. I checked the pressure regulator, and it appeared to work fine--pressure would drop when a vacuum was applied, but I wasn't sure if that alone meant it was good.
Long story short, I got a regulator and a new pump. Installed the regulator first to see if it fixed it and it did not. After installing the new pump (thanks Toyota for not putting an access panel under the seat big enough to remove the pump) fuel pressure was 43ish and no longer dropped when snapping the throttle. Took it for a spin and hesitation was gone.
Hopefully this helps someone.