It is my understanding the XJ came with these combinations

-- 3.07:1, manual transmission, I6 engine.
-- 3.54:1, automatic transmission, I6 engine with DANA 44 rear differential.
-- 3.55:1, automatic transmission, I6, V6 engines; manual transmission, I4 engine.
-- 3.73:1, automatic transmission, I6, Tow Package, UpCountry Package.
-- 4.10:1, manual transmission, automatic transmission, I4 engine.
-- 4.56:1, automatic transmission, I4, offroad or tow package.

Generally, the acceptable RPM range is 2000 to 3000. The optimum RPM range is 2300 to 2600. (based on 55-60 cruising speed.)

Either 3.73 or 4.10 will give you the towing capacity. With over 30" tires, I would opt for 4.10. Under 30", I would opt for 3.73.