I have a carrier with 2.73 gears (87 LG4) and since the eaton doesn't fit waht would be the next best one?

 

New rear axle with a limited slip that works. Not many options with 2-series gears and 26 spline axle shafts.

 

Where should i get the new unit? What years came with it?

 

Any year 82-92 Rear axle (used) . Search for one with the gear ratio, limited slip and the brakes that you want.

9-bolt with 2:77, 3:27 and 3:45 all with limited slip.
10-bolt with 2:73, 3:23 and 3;42 check for RPO G80
These are 3rd gen rear axle assy's

 

Powertrax locker and any ratio new gears you want(or your stock gears). Just get 2-series gears. For example, I'm running 2-series 3.73's with a powertrax locker using the stock open carrier--no problems for around 7 years. The locker is very streetable. Makes a rachet sound doing short corners. A junkyard posi could easily be worn out, or nearly worn out.
Also, auburn makes a posi that will fit, 2 series or 3 series. From Jegs....
109-542057 76-88, 3.23-Up $329.99
109-542058 76-88, 3.08-Down $329.99