high mileage drivetrain worth the swap?

here's my situation. i've got a 90 MJ 4.0/AX-15/NP231 with 155K miles that's in the process of being updated for a DD/work vehicle. i bought a 97 country edition with a blown motor and 296K on the ticker. i've swapped in the dash, power equipment, and the front clip, plan to flush out and swap in the open loop radiator, and i'm looking to get a utility cap for the bed to finish it off.

Here's my question. I think the 3.07:1 d30/d35s in the MJ are hurting my fuel mileage. I would very much like to have the 3.55:1 d30/8.25 axles out of the cherokee, but they have almost twice the mileage on them. Is it worth the trouble? neither set of axles have any posi's or lockers in them, and i don't plan on putting it through real hard use(occasional towing of no more than 2000#) Also, will the chrysler 8.25 out of the cherokee bolt right in to the rear of the comanche? i realize i will have to use the proportioning valve from the comanche.

Part II. I noticed when i was in the shop today that the exhaust manifold/head pipe of the 97HO motor looks like it flows a lot better than the 90 Renix pipes. Is it possible to swap them and will i see much difference from the swap?

thanks in advance...

I'd crack open the axles and give the components a visual, look for chipped teeth, etc. The gearing will give you a good difference in power, and while the 3.07's should net better mileage, they wont if you have a heavy foot thats trying to compensate for bad gearing.

The 8.25 spring perches will be on the wrong side of the axle tube since the MJ is spring under and the XJ was spring over. Other than that, you should be fine using it.