im sorry. but any claim to 31's with 3.73s is BS. From personal experience, 3.73 had trouble turning 235's
(could not run 5th) so you have got to be running the shit out of that truck.
For 4 bangers, 4.10 for 28"-30" 4.56 for 30"-32" I would not even go 4.56 in the 7.5". Which in most cases, the 4cyl will have. Your pinion is getting pretty small at 4.56 for that rear end.