L’Estrange, who signed from Manly Sea Eagles, is entering his third season at the club, while Diskin moved to Odsal from local rivals Leeds at the beginning of last season.
Bulls coach Mick Potter said: “I am really happy with these appointments. Matt Diskin brings a calm and calculated approach on the field and is a thorough professional, whilst Heath is very enthusiastic with great leadership qualities.
“By naming them both we will have a natural leader, in the position that touches the ball the most, on the field throughout the game.”
Bradford were captained last season by prop Andy Lynch, but he left at the end of the campaign to move to Hull.
Earlier in the week, it was confirmed that the RFL had purchased Bradford’s Odsal ground, which is believed to help secure the club’s immediate future.