JCB 1400B 4 wheel drive may not be working, any id

For some reason the 4 wheel drive on my JCB 1400B 4 wheel drive may not be working. The front driveshaft turns so I know the t-case is working but the front tires arent spinng properly which is causing me to get stuck where normally I wouldnt. I have lifted the entire machine of the ground then put it in forward and reverse to see if the front tires are spinning...they do spin some but not nearly as fast as the rears...could cold weather be causing the problem?

The front axle has plenty of lubricant and doesnt make any grinding noises, any ideas what the problem may be and where I should look first? I really want my 4 wheel drive back :)

Thanks for any advice.

GN

Thanks for the info. I did exactly as you said, put the backhoe in 4 wheel drive, lifted the front tires off the ground and kept the rears on the ground. I stuck a metal rod in the u-joint of the passenger side tire and was able to spin the drivers side tire forward and reverse(the front axle shaft turned as well) I then tried the same thing on the passenger side tire and was able to spin it forward and reverse as well and the front driveshaft spun as well.

Any idea what may be broken?

Thanks again for the advice!