91 xlt losing power

135k miles, 4.0 liter engine, autotrans, 4x4

When driving on flat or slight inclines it runs ok but any hill it slows down and feels like the engine is reving but not producing any power. I would compare it to a vehicle that had the vacuum advance line unplugged from a vehicle of that vintage.

It has new plugs, wires, no vacuum leaks etc. Starts right up and idles great. No check engine light as well.

MAFS is good, AIC is good, air filter new, recent radiator, hoses, fluids. thermostat replaced.

Any thoughts?

Thanks