I have a 98 dodge ram 1500 with a 360 the check engine light is on and the code i get

[fordluvr22]

is PO420 catalyst below? minnimum threshold (bank 1) what do i need to replace to fix this problem also when i have it in 4 wheel drive and take it out the light will not shut off right away and after it does it will flickerr onand off.