Ok, we had a very snowy winter, my (1978?) 245 was perfect all winter. Come Spring it was time to fill all the pot holes in my 500 ft long gravel driveway with fresh gravel. It has a frontend loader and doing the gravel was going great. So I'm on my 3rd or 4th bucket of gravel, idling so as not to slosh gravel out, in 1st gear/high, feet on running boards, not shifting, breaking, turning, just idling straight down the driveway, suddenly a big "THUNK" sound and immediately the back tires lock, bringing the tractor to a dead stop, killing the engine. I push in the clutch and start it right back up and, still in 1st/high, let out the clutch slowly and engine bogs down but no movement whatsoever. I try reverse, nothing. I try all the gears high and low, nothing. I put it in neutral and start to get off to look around, leaving it running, but when I start letting off the clutch it bogs the engine just the same as in all the gears. I turn it off, get off the tractor and look around and notice the skid marks in the gravel from the back tires are identical, about 8 inches long and beginning and ending identically. So I got my son with my pickup and a chain to see if pulling it back a bit might free up whatever is jamming it. We hook up and I get on tractor, start it, push in the clutch and my son pulls gently with the truck. It rolled just fine with the clutch in so we moved it off to the side of the driveway. BUT still if you try to let out the clutch and drive it normally it will not move at all in any gear or even neutral. It will only roll if you hold the clutch down, backwards anyway, I haven't tried to roll it forward. I jacked up the rear end and both tires spin: one in neutral, both in opposite directions when in gear, no funny noises or anything. I live out here in apple country in WA state. There are farmers and tractor folks all around me. Have talked to several tractor savvy people since this happened, all are scratching there heads. Help!