Speedometer cable on a GM automatic. Appears to be leaking at the inner seal, which seals around the shaft of the speedometer drive gear.
To repair, jack up the rear of the vehicle, leaving the front wheels on the ground. this will allow most of the fluid to flow to the front of the tranny. Disconnect the cable by unscrewing the retaining nut. Place a drain pan under the tranny and remove the bolt (with the cross in the head) and clamp. The gear housing should then pull out. If it doesn't put the cable back on and use a pair of pliers on the retaining nut to turn the housing without damaging the threads.
You will find two o-rings on the housing, one inside and one outside. Replace them. Also, inspect the speedo gear shaft for a groove worn from contact with the o-ring. You can usually get the parts at an auto parts store, go to a tranny parts store if there is one near.
When you re-assemble, use some ATF on the parts to lubricate them so you don't damage the seals.
