98 Jimmy engine management problems

Help Please. 1998 Jimmy 4.3l. Car ran perfect except for low fuel pressure when I took it into shop. Had fuel lines & fuel pump replaced. Had exhaust crossover and 02 sensors replaced. Now it refuses to run properly. I have a low reference voltage at virtually every sensor. I have replaced TPS, EGR, MAP, MAF, all to no avail. I have also replaced ECM with no improvement.
Right now I have code P0107. I am desperate for a solution, so I can have my car back to drive. The shop had it for 2 weeks, then gave it back to me in the condition it is in now. Even though it ran perfectly when I took it in, they finally gave up on fixing it and brought it back. Legitimate suggestions only please. Thank you