What model transmission is in a 1991 gmc sierra is it a 4L60 or 4L60E?

The 1991 GMC Sierra typically came with the 4L60 transmission, which is a non-electronic version. The 4L60E, an electronically controlled version, was introduced in 1993. Therefore, if your 1991 model has the standard automatic transmission, it is most likely the 4L60.