It sounds like the F-R synchros are bad. This is a simple fix. This first thing you need to do is pull the front transmission drain plug and see if you have any keys that might of fallen from the synchronizer. If you find one on the magnet you will have to remove the hood and side panels,brake and clutch linkage, electric connections, shut off cable, plastic dash, F-R roll pin, steering lines, and then you can lift the dash assembly with a shop crane. After this you can remove the 4 bolts that hold the F-R shift fork cover and check the gap between the Gear 24 and the thrust washer which on the old DK45 was .024mm. Here you will see the brass synchonizers and you may see them broke or find missing keys or have a bent shift fork.
If you find a problem with the synchros then just drain all of the hydraulic oil, disconnect three hydraulic lines on the right side, remove the right floor board bracket, slide back the 4x4 shaft cover and now get some jacks. Make sure you are carefull with the jacks because of course the tractor could fall crushing a part of your body. Once you have the tractor up on jacks and and are careful you can remove the bolts from the bell housing and slide the tractor back about 3 feet apart.
Secure both tractor halves with jack stands. Go to the front of the transmission and remove the top 4 shaft cover bolts and bottom 4 cover bolts. Turn each out of the way so you can remove the reverse shaft on the left. Once the reverse shaft is removed you can pull the top shaft cover and remove the bad synchros and keys. Take care when installing the synchros to make sure the keys stay in place and do not fall in the trans.
After you install the synchros install the top shaft cover and measure the gap. You may have to remove the shaft cover a couple if times to get the right gap between the Gear 24 and thrust washer. Once you get this done you can start to reassemble your tractor. It should take you 16-18 hours the firts time but if you are a dealer you will only get paid about half of that.
Good luck and I hope this help you fix your tractor.