91 xj problems

My 91 xj shudders and hesatates under a load. I already Changed plugs, wires, cap, rotor,fuel filter. so After I change the crankshaft position sensor.what is next like I said I want a 6in. lift 4x4 I want to get the most power but still maintain good mpg. of course I will get all the other things winches & snokel. I want show and go. And I also want to know where to get a wiring harness cause i think it needs one the wires are cracked and falling apart do I go to a junk yard or buy new and where to buy and is there any other jeep and years I can use.