I heard by word-of-mouth that the walk-in clinics are no longer allowed to issue the medical letters required for a driver's license and only the hospitals do this now. I hate to wait for hours at a hospital. Does anybody know if this is true? If true, what might be the fastest hospital?

The question is what License have you got now ?

on there check list website states: No medical certificate is needed for renewing a 5 year driving license.

Got mine at the local clinic for my 5 year renewal last week. Some agencies do require a medical certificate for 5 year renewal.

If you do need one, just go to one of the private hospitals. I've never had to wait very long at Sriburin for anything, and I doubt it'll be much more than a few hundred baht.

The hospital in Phayamengrai might work for you but I would guess there is one even closer, perhaps on the 1020. Thoeng is also a good option for getting your license if you don't want to go into town.