Power loss after certain MPH..?
 

It seems to me like this thing (2000 model, 2 door, 4x4, 168,XXX, new plugs) runs perfect, hills or not, until I get to like 45+ mph and then it wants to slow down to like 35 where it can comfortably pull strong in 2nd gear. Okay, if I try to keep up my speed, say 55 MPH, by giving her some fuel and staying in overdrive or 3rd either one, it starts to miss and gets progressively worse until the whole thing shakes and the SES light comes on with the random misfire code. I've searched this and seen a few posts with the same problem but no proactice answer. Anyone ahead of this?

tl;dr - Blazer loses power after 45 MPH
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P0300 is almost always caused by faults in the secondary ignition system: plugs, plug wires, distributor cap & rotor, ignition coil. Low fuel pressure can also cause it and it's free to check it.

Check the fuel filter for restriction and replace if necessary.
Check fuel pressure and leakdown. Key ON, engine OFF, fuel pump running, pressure must be 60psi to 66psi and must remain above 55psi for at least 10 minutes after the pump shuts off. Post your results.

I'll do this tomorrow, thanks. Now you say it's free, where should I go do get this done? Advance, Autozone?

Both of them loan/rent/sell fuel pressure testers. They're about $40 and it's a very worthwhile tool to have in your tool box if you own a fuel injected 4.3L engine. Fuel delivery problems are pretty common ;)

Unless you just replaced the fuel filter, might as well get one and replace it, then you know it's good. They're under $10.

I changed it 2 weeks ago when I had the gas tank off and on the ground 3 TIMES! Trying to fix the "gas gauge reading past full" problem. Spoiler alert: It was the lousy arm on the sending unit. I fixed it myself to where it'll never tear up haha. That is, of course, unless the pump goes...

Check fuel pressure and post the results, then we'll go from there.