Using a boulder wall as an 8' retaining wall (Dallas area)
The engineering on such a wall may prove difficult. We recently built a 6' retaining wall and a couple of 5' retaining walls. The 6' wall was 32 MPA concrete spray using our pool as the footing, the 5' walls are steel reinforced, core-filled besser blocks with footings 600mm deep and 1000m wide. In both cases, using steel and concrete, the engineering was complex. We rendered both walls. We constructed our walls this way to minimise the foot print (the walls are 200mm thick).

Using something like boulders could prove tricky unless you have a 45 degree angle of repose - this would simplify things but also eat up a lot of your back yard. 

My neighbour has just build a shorter retaining wall with besser blocks then added a rock finish afterwards - looks rather nice.