Acceleration on highway poor any ideas?

I was just looking for some ideas on what to do with my 98 XJ with 177k on it. l have maintained her very well and have had her for four half years and she has not cost me a dime more than regular maintaince and the mods i have done to her myself. I have a 4.5 RE lift runing on 31x10.5 mickey tompsons mtxs with an arb front bump with a warn 9000lb winch. As for engine mods I replaced the manifold with a banks tourque tube and a 2.25 custom exhaust with a dynamax muffler. Other engine mods have been the spacer and a cold air intake. The problem is when I get on the highway gas milliage is about 5-5.5KM/L around town and about 6.5-7Km/L on the highway and its always kicking down into 3rd gear it seem to be working to hard I only drive at 110Km/h but have been reduced to 95-100km/h and she can not break 120Km/h. I still have stock 3.55 gears with dana 30,35 so it may be gearing or maybe O2 sensors. I have never had to change them, but have pulled them and inspected them on occasion no engine light triped. The jeep starts good even in th -40 stuff and runs fine. Just not to sure I thought maybe with the added hight and weight that that kind of performance is normal so I am just looking for some feedback on any ideas or some reassurance that this is common.
Thanks Rusty

Ya I now its not
a sports car any more.

You've pretty much answered your own question. The added height, weight, and taller gearing due to the bigger tires and lift have robbed you of any semblance of performance on the highway.
You really have two choices to cure the problem (well maybe three but I'll come to the third later ). The first is to return to stock height suspension and tires but you might not want to do that. The second is to regear the axles to numerically higher ratios. Normally I'd say 4.10's with 31" tires but since you also have a 4.5" lift, I'd go for 4.56's. You'll notice a huge improvement in acceleration through the gears and be able to maintain overdrive on the highway.
The third? Well, the second option allows you to multiply the torque that you already have from your 4.0 to the wheels so the third option is to increase the torque produced by the engine. How? Either strap on a supercharger or build a 4.6L stroker.
___________________________________
Dino's "Mean Green Machine"
1992 XJ Laredo 4-dr - 4.6L I6 HO Stroker - 181k miles - AX15, NP231, D35c, D30

263hp@4900/321lbft@3300-3800, 1/4 mile = [email protected], 0-60 = 5.8secs
Websites - Jeep 4.0 Performance, 4.6L Stroker Build-Up, Dino's Jeep Tricks

Thanks for the advice. I guess i will have to live with what i have created.
Rusty