I have had it happen too
I had fifth gear fall off this last june. It happens without any warning, acts just like it slips out of fifth, and you can't find it back. Don't be afraid to drive it home, or even to the shop to get fixed. I had mine transmission rebuilt by Zumbrota Gear, in MN. They rebuilt my transmission with new bearings, syncros, a fully splined mainshaft, and fifth gear with a special locking washer that goes behind the spanner nut. They say this is the secret, even a fully spline gear and shaft, it will still come loose over time. They dyno test all trannys they rebuild and have a one year unlimited mile warrenty. They are one off the best companies I have ever dealt with, I dropped my transmission off on a friday night, and picked it up on saturday at noon, they also have rebuilt trannies for exchange and new modified trannies for exchange or sale. I would recomend them to anyone. They mostly do OEM rebuilding and manufacturing, the two guys that run it drive a bombed Cummins and a PSD, they know their trannies. I have had no problems with mine.