Someone had a good write up with photos of replacing the O-ring in the forward clutch last year I think.
Our 310 JD backhoe is fine when it cold but after working it for a while getting the forward clutch pack does not want to lock up well especially in high range but after I changed out the hydraulic fluid three times (due to blown hoses in part) and the filters twice it got better. When I first got it there was a lot of water in the fluid I had to back it up to the house once because it would not pull in low gear but now even hot it works well even pushing forward in low range in gears 1-4. If I am traveling a long distance on the road I will run in 6th or 7th if the fluid gets too warm to keep pulling.
In one case I read about here it was the worn/broken pin in the shifting linkage that was a simple fix.