Yea, that's not right... I have the same set uo, but with some smaller 30's on mine for now. I can pretty much hold 70 on the highway in 5th with the same exact set up... I even hit 85 once (once!) But she's a bit slow, but I am fine with that really... she's not a race car! However, I have no problems keeping up with cars driving around me.
For what it's worth, I am also running a 4.0L TB and a 2.5" PaceSeter cat back exhaust. Those two mods did help quite a bit with the highway end of things.