Odd Problem
 

Since I have bought my blazer it has has one recurring issue: Every time i fill the gas tank to FULL it will, soon after driving on a highway for a few miles, start dropping rpm all of the sudden. The best way to describe it is when i am cruising doing about 50-70 (roughly highway speeds) Ill be at around 1800-2100 rpm. All of the sudden the truck starts having "hiccups" where the rpm will drop anywhere from 100-700 rpm for a split second and jump back up. The most recent event was when i filled it up before a few hour drive and it started doing this for about 46 miles, i stopped at a rest stop to get a drink and when i started it back up and ran it, it didn't "hiccup" anymore. I have had the fuel pump replaced, tune up, fuel filter, gas cleaner, etc but nothing has stopped it from having these "hiccups" right after filling. Any one have any similar issues?

Here is a video of gauges: Yes I realize it is very orange... Truck Malfunction - YouTube

Can you actually feel a drop in power? It may just be your tach acting up. However, what brand of fuel pump did you put in and what did you replace with the tune up? I would still do a fuel pressure and leakdown test. Another member had the same issue you were having and it turned out to be a problem in the tach.

The truck will literally drop in power, when doing 65 it felt like I was hitting the brake every time it dipped, the truck is at the shop right now getting looked over, but once I find the receipt I'll get the make of the fuel pump.

Start with the fuel pressure and leakdown test. Could also be the throttle position sensor.

Connect a fuel pressure tester and drive it, see what fuel pressure does when RPM changes. Watch the gauge very closely to determine if fuel pressure drops and then it acts up, or it acts up & then fuel pressure drops.