Thanks for all the helpful comments!
I got around to setting the timing which was at 4 degrees and I set it to 5 degrees, and I started using Shell 93 octane fuel, and I also did a saturation cleaning to the M.A.F, throttle-body, and intake which was REALLY FILTHY! The truck seems a lot more at ease at 70 MPH now, but honestly she still seems a little wound up on RPMs. I'm beginning to think this is just the best to be expected all things considered.
So now this brings me back to the differential and automatic transmission question: is there an automatic transmission OR a differential with a lower gear-ratio number from a different vehicle that would go in without a lot of customization that would bring the RPMs down at highway speeds?
I understand my acceleration would suffer, but I'm willing to do some upgrades to the engine to get that back.

 

I'm thinkin 3.58 is about as high as you can go on gearing. Is your overdrive kicking in? You might be tearin around in 3rd, or the TC lockup may be inop. (If it has a Tq converter lockup, I'd think it would) What size tires, or did I miss that somewhere?

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Also, I'm trying my best to track down a gauge cluster with a tachometer but apparently they are few and far between that will work in a 1988 2WD automatic. There's one on e-bay but, bless it's pea-pickin heart, it's a train-wreck
I DID set my RPMs to spec using my stand-alone tachometer but I'm not going to go drive 70 MPH on the interstate with all that stuff set up under the hood, LOL!

 

Stock tires:
P195/75-14
Stock wheels:
14"x5" - J

Yeh I'm makin sure I'm actually in overdrive. There's a little button on the tip-end of the transmission gear shift handle on the column that turns it on and off. It definitely shifts down when you switch it off and resumes overdrive when you switch it back on.
Thanks!

 

Until you get a tach you really can't know. But I will say when I got my 93 Toyota I was used to driving my 1500 Chevy and the first time I went through the gears I "had no power". But that's because I was shifting around 2300 rpm's because that's what my Chevy does. Now I shift around 3200 rpm's and seem to have better power and still get my mileage. So it will probably seem wound up. A lot of people consistently wind them out too 4000 rpm's before shifting. With no reported problems. So these motors need some rpm's to get in to the power. As far as dropping your final drive speed you will give and take. If you put higher gears in it going from say 4:56 and dropping to 4:10's. your freeway rpm's should drop but you will be underpowered getting there. So it's a trade off.

 

Yeah, I saw the tire size you posted, AFTER I asked a dumb question. That sounds a little small... I think that's what my wife's Aveo runs for tires. Does it say on the vin tag what tire size? You'll definitely drop RPM's going up in tire size. Top speed/power? I don't really know. I know there's guys that stuff 30's on 2wd trucks. Don't know how well it works.

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