I have a 2006 F150 with a 9.75 LSD rear end. I know the clutch packs are gone. My question is, if the clutches are shot, then can it cause the rear end to slip while going straight. kinda like it slips out of gear then goes right back in.

 

The 2002 Screw I had would act like you ran over a pipe and the wheels turned but the truck didn't go anywhere. Only happened at slow speeds for mine. If that's what you have, it most likely needs the yoke greased on the driveshaft. A simple fix really. It does require a special grease so I would suggest getting a dealer to do it. The last time I had one done they didn't even bother to charge me for it. It's a simple put it on the rack, a shot of grease and hand you yer keys. Hopefully yours is a simple fix like that.

FWIW, the way the gears are in the rear there's little chance of them slipping without grinding. If you have grinding, it's going to require a set of gears and pinion seal.