Black grease coming out of u-joint

Since the truck was new the grease coming out of the transmission output yoke on the front u-joint is black on both cups. The old grease from the cups on the axle side of that u-joint comes out red just like the rest of the u-joints on the truck. I grease the u-joints every 10 days of work. More in the winter when it rains a lot. I am using Chevron Ultra-Duty Grease EP NLGI2. A neighbor suggested I use a lithium grease. The u-joint seems to be tight. I did have a warranty repair for a leaking output shaft seal leak when the truck was brand new. The driveshaft also had to be disconnected to fix an air line leak on top of the transmission last year. The truck is a 2 1/2 year old Volvo VHD D-13 with I-shift dump truck. Is heat from the transmission cooking the grease? Should I use a different grease? I was thinking maybe the mechanics might have left out a roller bearing when they reassembled the u-joint but the black grease is coming out of both transmission side u-joint cups. The red grease comes out of the axle side cups way before the black grease comes out of the transmission side cups.