Jack one wheel up, see how much you can turn it. With all wheels on the floor, how far can you turn the prop (handbrake released and out of gear) make sure it's secure and won't run you over
In my 1972 series III whenever the gearbox is in gear and engine at idle the vehicle consistently jerks forwards over and over instead of smoothly providing drive. It does this in both high and low range meaning my foot has to be on the throttle...