Is it a 5-spd or auto, if auto that mpg is about right.

Have you looked at the beaks, you could have a sticking caliper or rear shoe that is causing a drag on the drivetrain.

You could also be running rich, and simply wasting fuel. Check for vaccum and exhaust leaks. You might also want to check/adjust the valves, a loose one will hurt power/performance.

What is the age/condition of the spark plugs/wires, and the cap & rotor, it might be time to replace those as well.

More than tread lightly. Leave it like you were never there, nor anyone else.
'90 X-cab 4.88's 33 BFG AT's, rr ARB, Headers, Ignition upgrade, cold air induction.
'91 X-cab 5.29's 315's BFG MT's, rr ARB, custom bumper and flatbed