I have a 1991 f150 5.8 auto xlt. It has 4 studded tires. In four wheel drive it stops great on ice but in two wheel it slides badly. could this be a rear drum issue?
Thank you in advance

 

Could be the driveline is helping keep the front wheels from locking up too early ? and vice versa.

 

Exactly what Chris said, in 4wd the wheels are being turned mechanically and when the brakes are applied it slows the rotation, in 2wd the front wheels are being turned manually (by the road/movement) and when brakes are applied on slippery surface they lock up rather quickly causing you to slide. I have found that dropping gears in low traction conditions is much safer to slow down than braking (auto, drop from D to 2 while lightly pumping brakes).

 

This, wheels can't lock up from a downshift.

 

Your automatic will stop better if moved into neutral to stop when it's slick out, 2wd especially.