new holland tn75da trans slipping?

[pys1]

We have a 2004. We noticed after engine was replaced (cam gear cracked, pistons met the cylinders and metal through the bearings) a couple years ago that the trans seemed to slip. Thought at first it was the clutch (that the tractor doesn't have)... The symptoms are noticed more in Bh and C gear. another mechanic now says it seems to do it a little in Bl. Mostly going forward. It just seems like it takes the tractor a bit to get going. We only use it for snow plowing. Never seems to change with a load in front of it. Once it grabs, its going and wont stop until you spin out or hit the pedal. Brought it to the dealer and they say that pressures are good but more than likely it would be the high gear clutch pack and need to split the case. Quoted me 4-6k... Since new engine the machine has probably 500 or so hours on it. The problem hasn't seemed to gotten any worse. I changed trans fluid last year to see how it looked and checked filter and never noticed anything concerning and smelled like the new oil i put in it. any ideas?