I have a 75 JD400, love it -runs great, it's been very reliable and I heavily depend on it for manure management on my small acreage. Suddenly, I'm using it the other day and it mysteriously stops moving into forward or reverse. Checked the 2-speed control (thinking maybe I bumped it into Neutral, if that's even possible), it's fine. Tried both low and high, nothing. You can probably tell by now that I'm not very mechanically inclined but I did know enough to check the fluid. It was low (didn't register on the dipstick), so I topped it up - still nothing. So I drained the oil and replaced it with Hygard Low Viscosity, replaced the filter, etc (it was overdue for this). Still nothing. I checked to confirm the brake wasn't accidentally locked on, nope, brakes are not on. Checked the linkage, it does look OK to me - I don't see anything broken, but the hydrostatic lever seems fine, everything appears to be connected and it does move the rod that goes into the side of the transmission. I'm at a loss as to what this could be. Full movement in the FEL and the rear PTO is working (so I'm thinking it's not the hydraulic pump). I looked online at the cost of replacing it - OUCH, sticker-shocked, I'm . I'm back to the drawing board feeling determined to get this ol' girl movin' again It really is the perfect tractor for what I need. I've had a couple guys out here to take a look - much more skilled than me. Without investing alot of time and effort, we haven't been able to diagnose it--neither had tractor experience. I've been reading these forums and although there's a wealth of information on here, I can't find anything that helps this specific situation. I'm really hoping there's someone on here that has experienced something similar, or can point me in the right direction as to what to check. I'm willing to put a new transmission in it, but I'd hate to replace the entire transmission only to discover it was something else. I would appreciate any advice!