Kubota B2920 rear drive issues

[jjmjones]

My B2920 has rear chains on Ag tires. Using with a blade and bucket to move some snow. I am noticing a dramatic decline in the ability of the rear wheels to provide any help with moving the machine. It is in "4 wheel" and I am using the differential lock. The fronts are spinning like mad and rears do very little. I've tried to isolate the rears by switching to "2 wheel drive" and the rears can barely move the machine with little or no load. Nothing obvious to me is wrong. Am I missing something simple here? Trying to avoid the dealer.

 

[North Idaho Wolfman]

Check the air pressure, the rims are spinning inside the tires.
Fairly common issue in the winter!