The Dolphins might not be trying to move up in the draft to take a tackle instead of a quarterback.
They may be trying to take a tackle in addition to a quarterback.
According to Barry Jackson of the Miami Herald, the Dolphins have talked about acquiring the third pick which belongs to the Lions, without necessarily giving up their own fifth overall pick.
Going by a traditional version of the draft value trade chart, the third pick is worth 2,200 points, and the fifth is worth 1,700.
To come up with something resembling value without sacrificing five, the Dolphins would likely have to offer both their remaining firsts (the 18th worth 900 points and the 26th worth 700) and then some to get a deal done. Throwing in the 39th pick (510 points) gets it close.
Not every team uses the same chart though, and there are other models that would suggest giving up 18 and 26 for the third pick is an overpayment.
Ultimately, the decision will be made by the Lions, and whatever they might be able to squeeze out of someone else (like the Giants who pick fourth). Or, they might not want to drop out of the top 16 picks. Or, this might all be a smokescreen.
Either way, the Dolphins appear to be aggressively pursuing some major upgrades, which is the kind of thing you do when you strip down a roster for future draft assets the way they did last year. Having six of the first 70 picks and 14 total choices this year gives them the kind of flexibility to make bold moves.