Recently, my 93 suburban with a 91 cummins engine and 47rh trans seems to be having an issue but only when I try and go faster than 60mph. It will cruise around town just fine but once I'm on the freeway trying to go over 60(OD AND TC engaged) it craps out on me and I can feel a LOT of vibration in the gas pedal. Things chill out once I let off the throttle and cruise 55 mph or so, not sure what's causing the problem. I did use some diesel fuel treatment 2 tanks ago so I thought maybe I had clogged the fuel filter from all the gunk in the tank and lines moving forward so I replaced that, I also thought I might have had a weak lifter pump so I replaced that as well. only around 10 lbs of boost and 650egts during this problem each time. Still have the same problem, engine hates life over 60 mph any help would be appreciated

Specs:

first gen 91 5.9l 6bt engine swap no tachometer
stock h1c turbo with no intercooler
stock head bolts
Denny t fuel pin and boost spring
KDP fixed
3.75" exhaust, no cat or muff
stock injectors
47rh automatic transmission with TC switch and OD switch on dash
37" humvee tires