You shouldn't be using SPIKE relays to power the motors. The KOP (Kit of Parts) includes two Jaguar motor controllers, which use a scaled PWM input to turn the motors anywhere from +1.0 (full forward) to -1.0 (full reverse). Should you find that you require more than these two, additional Jaguars are available for purchase by FIRST teams @
http://www.andymark.com
Victor motor controllers are also an option, which provide similar functionality, but are not included in the KOP.
Both of these can be easily implemented in LabView. In fact, I think two are implemented in the default code. All you have to do is select which PWM channel they are using on the digital sidecar (typically 1 and 2).