The Sherman lever has more than likely worked loose and it can be snugged I believe by splitting the tractor. If you push or pull does it click into gear in low, neutral and high? If operating correctly it should have those clicks as it is engaged.
I don't recall anything special about the driveshaft but call Tony Jacobs about that and the differential and axles. e-mail me for his phone number.
The shifter lever for the PTO needs to be moved relocated down and to the left of the PTO cover where the Elenco chain/sprocket drive is now located. That is the PTO engagement lever. I replaced the rear center housing and had to dril and tap that hole to move the PTO lever.
The earlier Elenco 4WD's were full time power driven. Later versions had a 1" (give or take) nut located on the slip clutch/box attached to the PTO/lever inspection plate cover. Turn that nut to engage or disengage the drive shaft.
NOt sure what you offered truck, but it sounds like a parts rig and plan to spend some money for the front axle and differential and shaft. There should be some additonal stabilizer bars attached to the front hubs.
This is a start for you, but Tony can get you there faster. Tom M