Just installed 3 1/2 RC springs with 3 inch longer shocks, if I put a adjustable track bar in the front, what do I need to do to the rear, or can I just do the front?

 

3.5 rc springs are going to net you more than 3.5" - you will need an adjustable track bar front & rear or the brackets to support the changes in lift height. In addition you will need coil wedges in back & brake line extensions...
Considering what you will need to properly do this...an investment of that entire lift kit seems to make more cents (play on words). You can get that lift w/shocks, brackets & lower arms for less than $450 to your door from some supporting vendors...
Just saying....

 

X2. You'll need something to correct the rear tracking for sure (the wedges aren't included in all lifts of this height but are a great addition), but also some extended sway bar links front and rear. The RC kit comes with new rear sway bar links which you then will take your factory rear links and move them up front because they are longer.

The big thing to consider is what to do about your front drive-shaft. Depending on the year of your JK, if you are going to wheel it, you will need to deal with the fact that you may need a front driveshaft or at least an exhaust pipe extension.