You do know that if you just do one side and not the other at the same time, your ride off road will suck, right? What will happen is that your front end will be able to take hits so much faster and so much more comfortably that your rear end will constantly bottom out. While I have not seen anyone purposefully do this with bolt-on coil overs, I have known a few guys who tried doing this early on with the original EVO weld in coil overs up front and coils in the rear and it didn't take long for the guys in question to find the money to convert the rear to coil overs to match. If it were me, I would just wait till you can afford to do things RIGHT. But hey, it's your Jeep and your money and really, what would I know.