While the removal of the heat shield may not be helping, I doubt it's really causing the problem. The fact that you had a heat shield at all tells me you're running a '90 or older Jeep, and the fact is the injectors simply don't last forever. It's been covered here in depth in the past but I believe the expected life span is around 150K. Failure around the electrical connector is the norm. The problem seems to be accelerated by disturbing them, which you have by replacing those 4. This is why most people recommend changing all of them when one starts leaking. We had 2 leaking on our '89 last year, after removing the rail to replace them we had 3 more leaking. Wound up replacing them all and are now leak free.