02 mits montero 4wd xls "tranny probs?"

02 mits montero XLS 140,000mi had been parked for months now that its on the road again, i have noticed, in D gear there is a "shudder" upon take off or slight inclines, have noted it normally occurs under 2k rpm's, for now if i drop to D3, i havent noticed the symptoms. Concerned about high costs, Diamond III synthetic mineral oil, is just one of them. Considering dropping trans pan to check for deposists. IF clean, should i do a complete system flush/ trans filter replacement? If debris present what is next step?