engine problem or gearing?

ok i have a sort of curious thing going on with my 88. when i'm driving around town i have no problem with power or speed. but when i'm on the highway or freeway and hit about 45 or higher my jeep seems to be lacking power. i get into 4th gear (manual tranny I6L engine) and then into 5th gear and on level ground the fastest i can go is like 55. down hill its considerably faster. but uphills i lose a ton of power. any ideas on what it could be. engine problem or maybe i'm really low geared. i bought the rig semi built and i'm not sure what has been done to it and neither had the previous owner. thanks for any help.

You probably have 3.07 gears, and if you have anything from 31s or bigger, you'll experience that lack of power. IIRC, the consensus is 4.10s for 31s, 4.56s for 33s, and 4.88/5.13s for 35 and bigger....everyone has their own preferences, but in general, that's how I've understood it.

How big are your tires?

my tires are 31's.