1993 GMC Suburban ujoints or tranny?

While accellerating, more so under a load, vibration coming though both pedals (gas and brake), steering wheel, everywhere! Once you let off the gas, all is great. I checked the u joints, they're both tight, solid, and no "play". However, coming out the rear of the tranny (shift housing, 4 bolts holding it the the trans itself), there is a trans leak. If I shake the drive shaft, there's play on the yoke coming out of the trans. There isn't any play at all at the differential, just the yoke. Does this require an overhaul of the trans, or is this a basic bearing/seal I can change in the driveway?