HD6E Locked up

Bought first dozer about a month ago. Been working fine till last weekend. While dragging blade in reverse a loud bang and now no movement. I have a couple of manuals and started reading. I found transmission with no fluid in it to speak of. I was able to get all three shifting forks back to neutral position and nothing inside seem broken. The beast will start just fine it just wont let me pull clutch to engage transmission without pulling down engine and dying. Even in neutral. My question is this, if throwout bearing is out in clutch would it create this scenario. I am going back out to work on it this weekend. I am going to pull the u joints to separate the trans from clutch to find out which side is the issue. According to my book the trans is supplied with oil from the steering clutch and brake hydraulic system from oil lines from brake valve. I dont see any such lines on mine. To me it looks like the trans is filled from the top to a side plug and is a sealed unit. I am going to ad trans type one fluid but book also says to use sae 10w. Anyone have a preference? I am also going to check fluid level of clutch assbly and pull top access cover to get an eye on the situation in clutch housing. I am an appliance technician and have to tell you I have a great deal of respect to whomever works on these dozers. Also shows me  they were built with pride!

before you tear into everything, just put the trans in neutral and clutch in and see if the driveshaft will move even a little bit. try to see where the binding is. if not, remove the driveshaft to isolate the two like you said. what is the serial number and does it have dry brakes? if it is an older one, the trans and ring and pinion share the same oil. sounds like the input bearing went south. post some pics of what you have. does it have the allis engine or buda?

Sounds to me like a gear seized to the main-shaft as you stated the trans was low and you had to fight the forks to neutral.  All manual trans gears win neutral will rock or twist with gear lash, check yours, any that will not move are most likely seized and just dissassembly will be a bear!

Thanks guys for the input. I will get the serial numbers this weekend, not written in the book I have for some reason. The driveshaft does want to turn when I try to engage clutch, maybe 1/8 inch at most. I will also try to figure which engine and try to get some pics of everything and post.  Thanks again for the help.

Thanks for all the help. I took off the ujoint and found it was the  trans. Had a fully equipped repair guy take a look and said the lack of oil did it in. While in Quincy he found a used trans. for us so is now rebuilding that. Hopefully we'll have it back and running within the week. Again thanks for all the replies and helpful advice. I am an appliance tech so if someone has a ? about anything of that nature just give a shout. 

I am not sure but the batteries should be positif ground 24volts

Per the AC /FA Electrical systems guide the system has two 12 volt batteries wired  in this manner.  Negative  post on the right battery goes to starter solenoid. Positive post goes to negative post of left battery. Positive post of left battery goes to master switch, which goes to  ground. This is used with a generator system.