We recently picked up an '88 Bayou 300, about a month ago. We finnaly got to take it out last weekend and it seemed that the diff lock was engaged wether I unlocked it with the switch or not.

I didn't think much of it because it was just a test run and we weren't out very long. Well today I went and checked it out and it had rusted itself in the locked position.
Now, I un-seized it and got it unlocked but I won't be able to try it out for a while. Will I notice any difference in speed? I know it will be easier to turn and handle better, but would the diff lock being engaged full time cause us to lose any speed? Sorry if it's a dumb question, but I just figured I'd throw it out there and see what you guys think.

 

The old bayou 300 2wd had an option of unlocking the rear diff which was mainly for protecting grass in yards when turning. You'r better off with it locked in most riding conditions as most every quad has a positive traction rear end (I think polaris has a utility bike with a TURF option similar to this). Anyway to answer your question, no it will not be any faster locked or unlocked. It will just spin and lose traction more when unlocked.