The e-brake light also serves as a sort of "brake system check" light... sort of.
What it's telling you is that there is a pressure difference between the front portion of your braking system and the rear portion, which usually means a leaky rear wheel cylinder or a leak in the line somewhere, or air in the system.
The RE 3.5" kit includes a longer rear soft line from the body to the axle... I assume you installed this and have no leaks, but did you bleed the brakes after you finished up? This is important, as cracking the system open by taking the old line off introduces air into the system, which will compress and soak up the braking pressure from the master cylinder.
Check for leaks and bleed the brakes. DON'T drive it on the road until you fix this problem.