I have a pressure relief vavle (Wilkins, P1520, 100 psi) on the main supply pipe that services the house. It's in-line before the pipe feeds into the water softener and then into the house. It has a slow drip coming out of it. The only replacement valves I can find locally are 75 psi. Could there be a problem with using 75 psi vs. the 100 psi?

Could there be a problem with using 75 psi vs. the 100 psi?
Since you've failed to inform me of the city main PSI coming into your home I can't answer your question. The valve you're replacing has limits of 50 to 125 PSI not 100 PSI. Could you resubmit your question giving me more details?
Regards, Tom