My 91 Renegade had a plastic tank. Someone probably swapped yours out for some reason at one point, or perhaps (as Jeep did in those days) they just used what was around that day. Any part-out 91+ should have a plastic tank you can use.
You need to start a new thread on the body repair issue. It's not easy to fix that, cuz there's no "support" way up in the corner. The tub on mine was repaired in that area too, but it still flexes. My '88 had teh same problem. My solution was to tack a short bolt between the roll bar and the striker area. WAYYYY better, nice solid feel.
'91 Frankenheep YJ. 4.0, 4" BDS, Richmond 8.8, Titan 12k, 35" KM2's, rust.
'88 YJ, 4.2/4.0 head, +4½" & shackles, BFG 33's (sold)