Might be the rubber cushion inside the driveshaft is slipping, it's rare but it does happen, if that's what's actually happening you will need to replace the driveshaft.
I would think if this was the problem, it would slip in all gears.
2009 Mesquite Brown GL1800 Airbag
2008 Pearl Alpine White GL1800 Airbag
1980 Suzuki GS850G
1972 Suzuki GT750
1973 Honda CB350