My first instinct is to say that they are interchangeable. But there is a slight difference. Ensure is more moderate, but still like a pledge to make sure that something happens. A guarantee has more of an emphasis on your promise with you facing penalties for not getting something done. A guarantee is also written documentation at times regarding a product working properly. I would say that the difference is not too overwhelming but guarantees can have connotations in terms of law.

Ensure make certain that (something) will occur or be the case. "the client must ensure that accurate records are kept" make certain of obtaining or providing (something). "legislation to ensure equal opportunities for all" make sure that (a problem) does not occur. "only by researching stocks thoroughly can a client ensure against being misled" noun 1. a formal assurance (typically in writing) that certain conditions will be fulfilled, especially that a product will be repaired or replaced if not of a specified quality. "we offer a 10-year guarantee against rusting" "all repairs have a one-year guarantee" 2. LAW an undertaking to answer for the payment or performance of another person's debt or obligation in the event of a default by the person primarily responsible for it. verb 1. provide a formal assurance, especially that certain conditions will be fulfilled relating to a product, service, or transaction. "the company guarantees to refund your money" 2. promise with certainty. "no one can guarantee a profit on stocks and shares" So a guarantee is to assure that something will happen while ensure is to make a pledge to (someone);