I know that persisted at least through the 1988 model because I remember having to pay extra to get a 20-gallon tank in my '88, and then having to also upgrade the tires (all the way up to a massive P205/75R15) to carry the extra weight of all that fuel.
Not certain, but I'd guess '89 or '90 was probably the last year for that idiocy.
J.C. Whitney is your friend. They have new gastanks for the XJ for a reasonable price. I wouldn't fuss with a used tank -- I put a used tank into an XJ for a friend, and 6 months later the "new" tank was leaking.