Rear wheels underpower while in neutral in a manual???
 

So I finished up my diff bushing change and got her put all back together and while still up on jack stands i go to do an exhaust leak test. Start up in neutral and let off the clutch and the wheels start spinning? Bad enough you could feel it shake when you hit the brake. If i hit the gas it would flash the slip light and cut the gas. So i finished up the exhaust leak test by pushing the clutch in so it didn't spin the wheels. Dropped the car down and did a test drive seems fine. Any ideas drove manuals my whole life never ran in to this before but never really started a car in the air. Thanks

You have some drag inside the gearbox ..........

Friction in the the main shaft bearing (which sits inside a hollow in the input shaft) is the root cause.

Will be worst when the gearbox oil is cold and viscous.

Nothing to worry about