Lonecountryboy, most problems with the fuel sender are due to corroded windings on the sender unit. The corrosion occurs over time, making it almost a certainty that a junk-yard unit that will fit your '88, will be as bad or worse than the one you currently have.
Trying to find an after market unit that will physically fit in place of your stock sender, and, will have the same electrical properties will be difficult, if not impossible.
I spent a lot of time trying to replace my '88 sender because it was very erratic; a good sign of the presence of "corrosion" on the windings. I could not find an acceptable alternative, other than a very expensive replacement from the dealer.
The local junk-yard insisted I pay him to pull the unit and refused to let me check it for electrical continuity, saying my only recourse if it didn't work was to return for another unit--he only had one XJ on the lot.
I chose to clean mine with CRC electrical parts cleaner. That was 3 years ago, and it's still working great.
Moral? Clean yours first, before spending time and money replacing it.
