Rear End Issues

I recently bought a 2004 Gt that has obviously had some work done to it. The seller (Third Party) claimed that it had cam work done to it, but he didn't know what. Ive noticed that there are mounting brackets where an intercooler would normally go. And it has larger injectors. But the thing that puzzles me is when i am driving on the interstate if i am doing 70-75 in 4th (3100-3200 RPMs) and shift into 5th the RPMs drop to around 1800-1900, which seems rather low for that kind of car. Ive been told several times that the rear end is not stock but nobody seems to have a clue what would be in it. This being my first Mustang I haven't the slightest clue. I do know that once i hit 5th gear the thing starts sucking gas like its going out of style. Does anyone have any idea what may be the issue?

ok sweet thanks, i just wondered since it seemed kinda low compared to everything else i have driven

thanks guys for the quick answers, i would like to start adding to the performance of the car but i really had no idea what i had to work with, now if i can just find out the deal with the cam i can start...haha. I've had five different people (All ASE Master techs with Highly modded mustangs) tell me that it has a "slightly bigger cam" cause they can hear and feel the lope in it, but no one knows for sure. But i do know it is fast a lightning, even impressed the guys as they said "way to be fast to be stock." The problem is, nobody will even talk to me about mods or tunes with out knowing what cam is in it.