have an 87 4cyl that runs great until i step on gas and put engine under heavier load, then it starts chugging and losing power, i let up on gas and it runs great again until i load it up.  it will not do this if i down shift first.  it normally does this at a steady speed and i start going up a hill or incline and increase the pedal for more gas.  

Check the spark plugs, cap, rotor, and spark plug wires.  Ignition miss is common umder load if any of these parts are worn out.  Also, the fuel pump could be weak or fuel filter clogged, causing fuel starvation under load.  My son had an Explorer that ran fine until he towed a boat.  It was his plug wires.

Good luck.  

mzdiode:
have an 87 4cyl that runs great until i step on gas and put engine under heavier load, then it starts chugging and losing power, i let up on gas and it runs great again until i load it up.  it will not do this if i down shift first.  it normally does this at a steady speed and i start going up a hill or incline and increase the pedal for more gas.

It's an 87 4cyl and probably worn out. Do a compression test. Shifting to a different gear changes the torque band were it can still make enough power under load.