'84 turbo 400 locked up, won't even roll in neutral

[hrc200x]

Truck is a '84 chevy 4x4 3/4ton 350 motor, 400 tranny, 208 tcase with boss plow. Probably around '05 I was driving the truck for the company that owned it and cuising down the road at 55mph it went into neutral, I figured tranny went out. Pulled over, messed with some things, tried 4 lo and it drove, so problem was in the tcase. Truck sat for 2-3 years at the company till I bought it. I was only going to use it for a plow truck around the house so 4lo was all I needed. I ended up trying 4hi because I didn't like how every shift bangs in 4lo and its been working for the past 3 years in 4hi. Yesterday I was plowing for 5 mintues and pushed up into a bank, tried to back up and it wouldn't go, gave it more throttle, somthing went bang and it drove backwards fine. I figured a brake was sticking because usually when I start out plowing a rear brake hangs up going down the driveway. Anyway, push into a bank and same things happens, give it more throttle, bang, back up just fine. Then I try to go forward and it took a ton of throttle to move, so I limped it up the driveway. Basically it goes forward in drive with more than usual throttle, it won't go in reverse at all. I thought maybe it could be brakes, so jacked up a rear tire, unlocked the hubs, tranny in neutral and tried rotating the tire, wouldn't go so took tire off and attemped to remove drum and it was fairly stuck because of the groove cut into the drum from the shoes, after much beating on it I got it off, backed the adjuster off and put drum back on, still the same issue, wouldn't turn over. Tried jacking up both rear tires at the same time and they each spun so brakes were fine, moved to the driver side front, jacked it up and wouldnt' spin at all. take off tire, pry and beat the caliper off, bungie it to the frame so I can go finish plowing and it still wouldn't back up. Check tranny fluid, its a quart and a half low, top it off and still no difference. After I'm fairly sure its the tranny here is what I know. Put shifter in reverse and RPMs drop on motor but it doesn't move, put it in neutral and give it throttle it lurches forward a bit, try to push/roll truck by hand in neutral and it won't roll. In drive truck moves forward, pull it down to first and it doesn't really want to move. I've tried 4hi, 2hi, 4lo and neither lets the truck move, obviously because its the tranny. Neutral on the tcase the truck will roll.

Could I be lucky and the problem is vaccum related? I know there is a vaccum leek under the hood. Fluid is good and red, doesn't smell burnt.