Will a cross member from a 1990 4X4 Bronco with the 5.7L engine fit a 1991 F-150 4X4 with a 5.0L engine?

There was never a 5.7L engine put in the Bronco. The 351W is a 5.8L engine. The crossemembers are identical between the Bronco and F-150 in the years you mentioned. Both the F-150 and Bronco sport Dana 44 TTB's front, their suspensions are largely the same.