Behind the rear axle there is a cross member with some attachment on it. Obviously it is necessary, my question is can I relocate it to make room for a auxiliary fuel tank? 99 std cab, flareside bed for ref. Thanks for your help.

 

Pictures here of the rear frame, should be similar to your 1999: https://www.ranger-forums.com/genera...cement-141907/

Its the spare tire "rack", and yes you can remove it and put in new cross members or cut it and re-enforce it

Thread here on putting a Bronco 2 gas tank in a 1998 Ranger: https://www.therangerstation.com/for...ranger.132155/