嘞 has two pronunciations, le1 and lei. It's a spoken word which is very similar to 喽 but different from 了.
了 is a tense particle which focuses on the aspect that something has already occurred, while 嘞 is a modal particle which focuses on the aspect of (positive or negative) acknowledgement.
Example of 好了:
晚饭好了么？Is dinner ready?
Example of 好嘞:
开饭啦！(Broadcasting) Dinner is ready!
Depending on the context, 好嘞 could also mean 'affirmative', 'well', 'there there', etc. 好了 doesn't have those functions.
In practice you may see people write 好嘞 as 好了 in written language. To me that's just a widely accepted misuse.
Bing Dictionary: link