If the rears work fine, I would try switching the front and rear wiring at the stereo. If the rears then cut out you probably have an issue with the pioneer. If the fronts are still cutting out try replacing the wiring with new speaker wire and check all your connections.
All the above suggestions are great. Especially to tighten everything. Also remember when the sound sucks compared to what they were in before, the front speakers are free air in the Jeep with no backing to help with the sound like in a box.