To all of you who have responded, my great thanks...
I believe I am closer to the simplest, least expensive, least hassle solution...not sure though. A recap here, with new info...

First, my vehicle: '99 Base model, 4cyl. Auto - NON-ABS - electronic speedo.
Original tires 225/75/15 - NOW riding on 265/70/16 (optional stock size).
Speedometer reading now LESS than actual speed...about 5mph @ 60mph.

Second...regarding some of the solutions offered, all of which I appreciate for your experience and time. My use for this truck is a simple daily driver...period.
I do not have nor want a GPS, do not intend to buy any additional gauge which I assume from photos has to be attached somewhere to the dash, and I have no intention of swapping diff. gears. Just want an accurate speedo...

I spent some time at a Yota dealer's shop with whom I have a good relationship. Fortunately for me, a slow day so I did get some useful info. The speedo sensor for my truck is fairly common across many T4R variants...primary part # is 83181-12020. On my truck, located on driver's side of the tranny, very easy access. The "driven gear" as they identify it is quite easy to replace, a simple retainer clip...and they usually come with a new oil seal as well. My guy at the shop had a few conversations with a tech who wasn't too busy...and after about twenty minutes, he came across a notation on one of these gears which specified "for 16" tires". Part # 33403-39415. Ah, success!...I thought...but came home to do some last verification before purchasing something I can't return. Now, not so sure, somewhat confused...

I came across a webpage called "Adventure Taco"...obviously an avid off-roader but also a serious numbers nerd...wow. What a load of info. But he did have a LOT of relevant stuff on there, including some other driven gear specs, and now this is what confuses me. All these driven gears identify as 33403-( ), with the second set of numbers designating tooth count. The 39415 variant is 33 tooth, the largest they make. But if my speedo is reading SLOWER than my actual speed...wouldn't a larger, more teeth gear make it WORSE? There are two other variations specified for auto trans., one 32 teeth and one 30. I do not know what is currently in my truck. Only way to know would be take it out and count 'em. Not going to buy anything until I get this sorted out. Still hoping this might be my easy fix...if not I'll have to go the SpeedoHealer route I guess. Hoping some of you who have commented are following this thread...anybody?