Used parts:
1. Swap rear end from parts car. Not sure which years are compatible.
2. Swap differential guts from a used differential. Again not sure which years will work.

New parts:
1. Get differential rebuilt. (Does this mean new gears or only new seals/bearings?)
2. Put in aftermarket differential (TrueTac?). Would new ring and pinion also be necessary?

Advice? Comments?

your options that make actual economic sense are:

1. drive it, accept the howl and 'eventually' it will die. this may be in fact YEARS from now. these dana axles are rock solid and incredibly tough

2. your 89 axle is a non abs axle which mean you must swap it with a non abs replacement. if you do not your speedo will not work. i would if i were you find a ""complete axle"" assembly.

3. you have an automatic the most common axles will have a 3.73 ratio try to find one from an 88-90 240 or a 91 with NO abs brakes.

4. if you have a manual then the ratio ought to be 3.31. try to find one from 88-90 5 speed donor car or a 91 5 speed with no abs brakes.

5. avoid any axle from 92 or 93 cars because of abs brakes

you can if you really want use a 3.31 axle for any auto car or vie versa. your mileage will be be better or worse and your acceleration will be better or worse depending upon which axle goes where.

you can use if you must earlier than 88 240 axles which have odd ball ratios not used in later vehicles.

rebuilding an axle requires tools, knowledge and a skill set not commonly found in most repair garages. the cost will be imo more than swapping an entire assembly from a junkyard. if you opt for this route speak with garages or shops that build race cars