What are you calling slippage? Also is it taking off straight or around a curve? Straight I'd lube the slip yoke in the driveshaft where it mates to the transmission as initial cushioning is gone after some use. If during turns, may have clutch issues
Another thing is many say rear end issues when it's a transmission not shifting correct. Ford has many bullitins about this
March, 3, 2011 AT 5:30 PM
A differential really can't slip. The pinion gear drives the ring gear which directly drives the axle. If the transmission slipps, you may think the differential is the problem.