Wow, you have replaced alot of stuff............stop that, lol.
How does it run at 70mph?
Better than at 40-60mph?
Is it a manual or automatic trans?
The most obvious is dirty fuel filter, you notice that first at highway speeds, but you have changed that.
So next most obvious would be exhaust restriction, again this first shows itself at highway speeds, Cats and mufflers can collapse inside, this restricts exhaust flow, as RPM/load increases you hit a point where back pressure in the exhaust system prevents new air/fuel mix from coming in so acts like it is fuel starved.......because it is.
Vacuum gauge can be used to test for this or feel tail pipe while someone revs the engine, you will notice a lag in tailpipe pressure at higher RPMs or no increase in exhaust pressure at a certain point.
Next, '96 still has a Fuel Pressure Regulator(FPR) on the fuel rail, it is vacuum operated.
A gas engine has high vacuum at low RPMs so FPR is sucked open and extra fuel is sent back to the fuel tank.
When RPMs are high, vacuum is low so FPR closes leaving more fuel pressure for the injectors, if FPR is sticking open then fuel pressure drops at higher RPMs starving engine for fuel.
Fuel pressure gauge would be used to check this.
All of these would get worse above 60mph not better.