Generally, yes. You can activate a different Verizon phone on your line at any time, and once the X2 has been deactivated, it will be available to activate on another Verizon account.
Where did you purchase your X2? If you purchased it from a 3rd party retailer, like Amazon, Wal-Mart, Wirefly, or authorized retailer store location, you may have signed a second contract directly with that retailer that prohibits you from deactivating the phone for 6 months. If you violate that contract, you will owe that retailer an ETF, even though Verizon does not restrict changing the phone on your line or selling a deactivated phone.
If you purchased your X2 directly from Verizon (this site, telesales, a corporate store), then you're good to go with your plan. Otherwise, check your paperwork and make sure that you won't be hit with an ETF.
Thank you spottedcatfish for your response. You are correct in saying that beactivelife can sell his Droid X 2 and activate another device on his line.
As long as your account remains in good standing you will be able to sell your Droid X 2 and have it activated on a different account.
Please keep in mind that if you currently have the, grandfathered, unlimited data plan that you need to activate another Smartphone on your line in order to keep the plan. If you activate a non-Smartphone you will be asked to change to one of the new tiered data plans and will not be able to go back to the unlimited data plan. Click here to view our current data plans.
Hope this information has been helpful.