It's got about 2500 hrs on it.  When we put it in gear and give it some throttle, it doesn't really try to move but it bogs the engine down. The back wheels turn easily with the machine jacked up, whether it's running or not.  I've had the shift fork cover off and manually engaged the transmission with the same results.  I tried the same tests with the 4wd engaged and that didn't change anything.  Any suggestions would be helpful.  TIA  J.R.