As previously mentioned, change the IAS housing and motor. The butterfly on these throttle bodies close completely at idle. There is a bypass passage above/below the butterfly that allows idle air around the butterfly at idle. The 2.5 housing has a smaller hole(to allow less air) than the 4.0 and the plunger on the IAS for the 2.5 is smaller to match this hole. Using a 2.5L IAS in a 4.0 housing will not work because the plunger cannot seal the larger hole effectively. This housing has Security torqx fasteners so you will need a special bit for them, if I remember correctly its T20 size. The security torqx have a little teat in the center so the bit will have to have a small hole in the end of it.
Some one dynoed the results of different combinations here a while back and there is a major difference if you bore out the intake manifold to match the throttle body.
I personally would recommend using a die grinder with a carbide bit to do the roughing out and using a sanding drum to polish it. I also recommend pulling the intake manifold off the vehicle to do so as it would be hard to keep all the aluminum from going through the engine even if you stuff rags down there. I laid a new tb gasket on top of the manifold, aligning it with the holes and then using a magic marker mark the inside diameter of the gasket onto the manifold. Grind out to that mark, beveling and smoothing everything back to the runners.
Also if you have it off you can pull the injectors and smooth/bevel the intake runners where they meet the head as mine did not match the intake ports at all. you will see where the gasket doesn't touch around the inside perimeter of the runners.
My throttle position sensor was a real pain to remove and would recommend squirting it with a good penitrant(not wd 40) and even a little heat from a small butane torch on the housing where the screws enter(be carefull, don't over heat.
My 91 has a MAP sensor on the fire wall so I removed the one mounted on the 4.0 throttle body and capped the nipple off. I think yours should be on the throttle body however(not positive)
My results were significant. I live on the windy plains of Indiana, where they have the largest wind farm in the US.
Before the head winds would keep me at 50-55 almost full throttle. Now I have plenty of power, have even caught myself speeding at 65 in the head winds without even feeling it. My MPGs have improved 1-2mpg as well. Last tank I got was at 23mpg.
While your at it I highly recommend removing the exhaust manifold, checking for cracks or missing/broke bolts and installing a felpro full intake/exhaust manifold gasket. Its available at napa. It will seal the exhaust manifold better and insure that your O2 sensor is not getting a lean signal from an air leak. The bolts/nut(remember to use a nickle or copper based anti seize) should be torqued to 23ft lbs starting in the center and working your way out evenly. Most important thing to keep from loosing bolts from the manifold is to go back and retorque those bolts/nuts after a few heat/cool cycles. I did require a crowsfoot to torque the front nut by the power steering mount but a dog bone would have been better. Remember when using a dog bone or crowsfoot that it most be oriented 90 degrees to the torque wrench to get a correct torque. Orienting it straight out will lower the torque. There are calculators out there that calculate the actual torque at the fastener based on the length of the dogbone/crowsfoot so you can set the torque wrench correctly to take this into account if you cant set it at 90 degrees
