How is your engine?
My '86 2WD Suburban with a 350 and 700R4 was topping out around 75 mph. I did some work on the engine - new valve seals, adjusted the valves (obviously I guess after changing the seals), spark plugs, plug wires, new dist cap and it sort of came to life. Not only would it go faster but the mileage improved quite a bit. Not sure if it was any one of those items by itself but whatever it was, it sure helped my top end.
