Truck won't go into 4wd

[cobrajet1]

I put my 99 f250 psd into 4wd and everything seemed okay, the 4x4 light came on I locked the hubs in (Warn) started to drive and the truck went sideways. Dropped the truck off at the dealer and the replaced the Front hub and u joint and brake rotor for some reason? Got the truck back and same thing so I checked the hubs and they seem to be locking in the transfer case is locking in and I even put into 4x4 low so I guess I broke something a axle or front diff not sure? I called the dealer and they think it's a vacuum problem but I changed
the hubs last year and the Warns are mechanical and I shouldn't need the vacuum right?


Thanks

[7.3eleanor]

You do not need vacuum when u have warn hubs. Put the truck in 4wd and see if the front drive shaft is spinning. If it is u have a problem in the front diff.

[7.3eleanor]

Sorry if the front drive shaft is not spinning u have a problem with the diff.

[NoRalPh]

How do you know the transfer case is "locking in"?

[dieseldan2005]

If the transfer case is locking in you can unlock your manual hubs and try to turn the front driveshaft. If you can't it means that the t case has locked in. The way to test to see if the the front diff is ok is to dis engage the t case and unlock the hubs and turn the front drive shaft to see if the u joints are turning behind each wheel. They should both turn because there is no resistance on either axle.