This is a tail shaft (output shaft) for a GM 700R4 or 4L60 automatic transmission. These come in different lengths for application. Some have longer slip yoke (drive shaft yoke) splines, some have the speedo drive machined into them (like yours) and others don't. Some have a reluctor ring pressed on the shaft just ahead of the spline for an electronic speed sensor. This one appears to be a 2-wheel drive with the longer spline. 700R4 and 4L60 transmissions were used in almost all GM cars and truck with rear wheel drive.

This is a tail shaft (output shaft) for a GM 700R4 or 4L60 automatic transmission. These come in different lengths for application. Some have longer slip yoke (drive shaft yoke) splines, some have the speedo drive machined into them (like yours) and others don't. Some have a reluctor ring pressed on the shaft just ahead of the spline for an electronic speed sensor. This one appears to be a 2-wheel drive with the longer spline. 700R4 and 4L60 transmissions were used in almost all GM cars and truck with rear wheel drive.

References:

https://www.ebay.com/itm/2WD-OUTPUT-SHAFT-LONG-SPLINE-700R4-4L60-TAIL-SLIP-IN...