This is a solution for the Full Garage achieve for those who have had this achievement glitch (even after the update). This solution is based on the solution of Pinapple04 (in the comments of Thrashballer, above) combined with the comments of others and my own experience.
Mine glitched as well even though I only began the game after the update that was supposed to fix the glitch. I only tried the process by Pineapple04 above. I did not clear cache or recover gamertag, so I can confirm those steps are not necessary. Just to be explicit, my steps were:
1. Go to "Help and Options"/"Change Storage Device"
2. Select different storage - I selected hard drive instead of cloud which I normally select.
3. Return to Stunk Park/100hp/Crimson vehicle selection screen.
4. Scroll through all vehicle using LT/RT, but not selecting any.
5. Repeat 1-4 until it pops. It took me about 4-5 attempts as others have reported as well.
I did not restart the game each try and I continued to select the same alternate storage device (hard drive) with each try. I also used the "exact money" glitch described above to purchase many vehicles, so that did not impair my ability to get the unlock later even though it glitched.
Also, I followed no special procedures during my regular gamplay and vehicle purchases, e.g. I did not play all modes, exit after every purchase, or purchase the last car not using the exact money glitch. I definitely purchased my last car (400K) using the exact money glitch and I went in and out of menus while playing/purchasing without any regard (because I didn't know I should have been worried about the glitch).
Regarding the unglitching process above, my theory is that each time you select alternate storage (even if you select the same device) it gives you a renewed opportunity to scan the vehicles for the achievement. (You can see the save icon appear each time you select alternate storage, so it's probably resetting something in the save file.) Sometimes it still glitches so nothing happens, but eventually it pops.
This is probably not a factor, but the time that I finally got the achievement to unlock, when I was rescanning the vehicles, for every vehicle that had more than one option I only pressed the trigger 3 times, so I would not end on the stock vehicle, I would end on the vehicle just before it returns to stock, i.e. the most expensive, before moving onto the next car. It's probably just a coincidence, but the next person who tries this process could test this to see if it makes a difference.
Hope this helps. Such a relief to have it pop for those completionists out there.