Based upon the following factors
- only 290 hours on the tractor
- hydraulic oil level is correct
- type of oil is correct
- loader will lift full bucket of dirt to full height
- new hydraulic filter
- linkage does not bind
- will not move in reverse but will sort of move forward
It would appear to me that the drive motor has failed. I have heard of this at least two times in the past. The output shaft of the drive motor is connected to a stub shaft inside and that stub shaft has been known to snap. The least expensive way to resolve your problem is to purchase a used drive motor from a 444, 446, 448, 4016, 4018 or 4020 model off of e-bay and install it. The alternative is to take your drive motor to a firm that specializes in rebuilding hydraulics and have them change out the broken stub shaft. These motors are fairly complicated internally and the work should be done by someone who is experienced. Cleanliness cannot be stressed enough. Depending upon how badly broken the shaft is, there could be loose shrapnel inside the motor creating more damage every time you try to use the tractor. You should shift the trans-axle into neutral and push it by hand or tow it to a spot where it can be worked on.
As you say, the hydraulic system is working perfectly otherwise. That rules out the pump and the pump coupler. 15W40 is a perfectly acceptable grade of oil. With a mere 290 hours, the foot linkage (banana plate) should have minimal wear. However, sliding under the tractor and examining the linkage while someone uses the foot pedal and shift lever is a good idea. Watch the spool that goes in and out of the travel valve body to make sure the spool is going all the way in and all the way out. It costs nothing to check that out prior to dealing with the motor, which I believe is the real issue. If you want a motor and cannot find one, send me a PM and I will connect you with a guy in Michigan who can probably help.