The 99 + intake would really only help on a 91-98 4.0. On a Renix motor, the intake ports won't line up correctly with the head, and the higher flowing manifold wouldn't really help any. Plus, even if you got the manifold for under $100, they're kind of a PITA to put on an older 4.0.
There really isn't much you can do to get more power out of the motor for even kinda expensive Those bolt-on supercharger kits, last time I checked, were about $4000. If you want more power, look at gearing.