This is actually rather simple but it took me a while to figure it all out so I figured I would share what I have learned. One way to check is to look at the VIN # on the transmission. I will post a picture below showing where the VIN # is located on the transmission if you don't know where it is. The 10th digit is the year the transmission was made for. The code goes as follows.
L-
1990
M-
1991
N-
1992
P-
1993
R-
1994
S-
1995
T-
1996
V-
1997
W-
1998
X-
1999
Another difference is the shape of the shifter arms on the tranny. The 90's have shorter arms while the 91+ have longer arms with more of a curve. The picture below shows the 90 transmission on the left with the shorter arm(red) and the 91+ on the right(blue). I will try to take better pictures within the next couple weeks.
The mounting bracket that holds the shifter cables to the transmission is also different. This is covered on the VFAQ site so I will just post a link to it.
Shifter Differences, 90 and 91-94
I also used to think that only the 91+'s had this mount/pedistole but its not true. Some 90's do some 90's don't. most all 91+ I've seen have had this mount, but i can't say for sure that 100% of them do.
One more side note, the
93-94 transmissions have a larger diameter input shaft for the axle shafts. So you need to match transmissions with axle shafts.
