Nat Moore is one. The guy was a great receiver, but the Dolphins didn't throw much early in his career, and he also lost a few years of productivity with David Woodley at QB. But, look at the monster year he had in 1977 when the Dolphins opened up the passing game. And look at how good he was with Marino, late in his career.
I think Tony Nathan was underrated. The guy was a great receiver out of the backfield, and he averaged 4.8 yards per carry for his career. Shula loved to use multiple RBs, and Tony just never got enough carries to put up big numbers. Look at his 1981 season. 50 catches for 452 yards, and 782 yards rushing (5.3 yards per carry).
Mercury Morris is also a bit underrated. He was an amazing big play back, averaging 5.1 yards per carry for his career. Look at 1973. He ran for 954 yards, averaging 6.4 yards per carry with 10 TDs. Again, Shula loved to use multiple RBs, and Kiick and Csonka took carries away from Merc. It's hard to say if Mercury would have held up under the bigger workloads, but it would have been fun to see what would have happened if he had a 250 carry season while in his prime.
Others who I think were underrated:
Ed Newman- 4-time Pro Bowler
Earnie Rhone- leader of the killer B's
Bob Brudzinski- a great run stuffer
Steve Towle- a tackling machine, 217 in 1976!
Vern Den Herder- a very productive player for a very long time