It's such a critical position that it's a rarity to see good ones hit FA and teams aren't willing to move them once they find one. Only reason Whit hit FA is because he was getting up there in age and Cincy had committed to moving on. Usually if a guy hits FA there's something wrong with him, i.e. the team doubts him for some reason to either play up to the contract or do the job.
Looking at this FA group you have two capable LTs in Tyron Smith (Dallas) and Trent Brown (New England). Both will almost surely be signed or there will be a tag 'n trade which will cost premium picks then you have to pay the guy on top of that. Not sure I'd want to pay either of those two.
Best way to get a LT is take one early in the draft and hit on the pick. Then you get five years control of a premium position at a bargain. If things don't fall for us that way in the draft we're probably better off continuing with guys like Jackson or Boom and try to get the rest of the OL playing at a high level.
I would go after that Center from the Dolphins if he hits the market. Him next to Avila would be fantastic, and then we have a guy at the pivot who can actually assist whoever gets the nod at RG instead of needing protection like Shelton. In the draft if a LT is there early you take him. If not then take speed outside the hash (wideout/corner) or an edge rusher. But ideally a LT will fall into our lap right at the top.