Straight Talk allows Verizon 4G phones on their network!!!
To be specific, I checked a few ESNs with their system as well as chatted with a person on this, and you can in fact bring over a Verizon 4G phone and use it with Straight Talk. This is CDMA ONLY, and you will be limited to 3G speeds.
I will be verifying this in the next few days when my new (used) phone gets here. If it doesn't work, I will be sure to let everyone know/delete this thread.
I was quite pleased to hear this because I was tired of having to look for 3G phones that are all outdated, and ST's smartphones can be expensive. I was able to pick up a near-mint LG G2 for less than ST's GS3. But, we'll see if it works!