I have a 2004 4WD Reg Cab Tacoma. I want to add a locker or LSD. There is a TRD on eBay 00646-41301-100. Will this work? Or can nybody help me on choosing a locker or LSD for my Tacoma?

A TRD locker will not fit into a NON-TRD housing.

It can be done but there is some grinding and welding involved heres a site that talks alot about doing this.
http://home.4x4wire.com/erik/diffs/#8.25

Can a TRD limited slip not a locker work on a 2004 reg cal 2.7 4WD?

According to some of the sellers of the TRD LSD on Ebay it will.