First... Welcome...

and
X about 9 on the freeze plug issue. Not very common on a 4.0l at all, hence the questions to you.

Define 'a little buck.. The AW-4 shifts in a little harder than most trannies out there. Its perfectly normal. I doubt that new fluid and a filter is going to do anything. The AW-4's "filter" is little more than a screen for bigger pieces.

As far a synthetic fluids in the tranny. Its a mixed subject at best. There are diehards on both sides of the discussion. But you don't know what was used and how the tranny has been maintained over its life. If you have conventional in it. Its best to stick with it.

Honestly I don't think even with mods you are going to get much past 20 mpg.
Its an aerodynamic brick, with an auto tranny and mostly likely 3.55 gearing. Run your tires near the max PSI rather than the recommended.

I can manage about 21 MPG with my Comanche with a 5-spd, 3.07 gears and rock-hard commerical grade 235/75-15 tires.

well i dont know when the last time the tranny has been serviced so i will look into geting it done asap...but i noticed that when put in gear it gives a little "buck" i dont know if its normal for the 4.0......and on the oil im sure that conventional was always used so ill stick with that. Also looking under the truck i see 2 O2 sensors??

You problem could lie in your Motor and tranny mounts. My Waggie always gave a lil buck and sometimes a thump when I first got it.

I replaces all the mounts and now she rides smooth. The tranny mount is sitting right on top of the X-member. But before you remove the X-member put a jack under the t-case to support everything. Tranny mount is about 45 bucks at autozone. There are two 19mm bolts that hols the tranny mount to the tranny these are prone to back out you may just have to add some locktite and torque them down some. HTH