dodge

check fuel pressure and check for codes,

Suspect the pickup screen in the fuel tank is collapsing and plugged. The engine will usually run ok for a few miles, then stall when the largest volume of fuel is being pumped, . . . which is during coasting.

caradiodoc

If the fuel filter was clooged It would give poor acceleration but it would idle ok 88 dodge forget codes.Is the truck fuel injected.Not sure If dodge started tbody injection in 88 or 89 for there pickups.If so check fuel pressure.also is there a rotten egg smell if so could also be clooged cat converter.according to what your explaining sounds like this is a carb engine and if so you might have to have the carb re built.

Not the fuel filter; the pickup screen in the tank. If it's a carbureted engine, that will indeed allow it to idle but not run well at higher speeds even as low as 20 mph. If it has fuel injection, a plugged screen will cause the engine to run best at highway speeds and it will typically stall when letting off the gas. The pump has to move the largest volume of fuel during coasting. When the screen is plugged or collapsed, that higher volume can't get through so pressure drops. When it drops low enough insufficient fuel will spray from the injectors, and the engine stalls.

caradiodoc