Brake light is simple, pull the bulb.

As for the speedo reading, I would only worry about it in HI range since that is where you do most of your driving. Hopefully you're only using LO range when offroading. The speedo is likely wanting some signal telling it that you're in LO range so it knows to use a multiplier to adjust the speed output. Even if you had a toggle swith on the dash it still may not be correct due to the fact you likely have a different gear range in LO with the 88 transfer case vs the stock unit calibrated for the PCM.