Which washer?
http://www.subaruparts.com/subaru.diag/2/i190-A.gif
Differential
01 $356.51 Gear set-hyp
02 $16.22 Pinion-diff bevel
03 $5.78 Wash-38.1x50xt t=0.95mm
$5.78 Wash-38.1x50xt t=1.00mm
$5.78 Wash-38.1x50xt t=1.05mm
04 $30.07 Gear-diff bevel
05 $37.27 Sft-axle drive
06 $0.23 Bolt
07 $0.53 Plate-ret lock
08 $30.00 Ret-diff side
09 $0.44 Circlip-inr29.8x2
10 $5.03 Oilseal-30x50x9 left hand
11 $54.78 Roll brg-40x80x19
12 $1.13 Bolt-10x39.5x15
13 $59.89 Case-diff left hand 1x- m/#358880-
$59.89 Case-diff left hand -1x
14 $13.72 Sft-pinion
15 $86.23 Case-diff right hand -2x
$86.23 Case-diff 2y-
16 $1.59 Str pin-5x40
17 $17.75 Gear-speedo-drv
18 $5.03 Oilseal-30x50x9 right hand
19 $3.82 O ring 84.5x2