Joe, I have an 86 Toy PU, 22R similar to your 4Runner. I can attest to Toyotas, as mine is at 215,000 miles and still gets around like a mosquito on sterioids...
Anyway I was a little surprised to hear that the truck will only do 70 on the interstate. Mine is a 4 cylinder also, and it will do 100+ mph down flat freeway, 85-90 up hills. You may want to consider doing a little tune-up to give her some more horses. I have put a Magnaflo Exhaust, Competition Cam and Weber carburretor on my pickup, and she is pretty dang quick for a little 4 thumper.
If you're looking at new Toyotas, like a Taco or a Tundra they are still very good trucks. My sister has a T100 and she loves it. Newer trucks are much faster, but probably won't get the fuel economy that your Runner is getting.
Just like the mechanic said, can't go wrong with a Toy! My dad and brother are both mechanics and told me the same exact thing many years ago!
You should take your runner four wheeling, too...it'll blow your mind what that little stock truck can do!
:-)
Lisa, Colorado