GPI Overhaul Kit (For EZ-8)Incl. Motor Shaft Key, 2 Gears, and Replacement Seals - 13750005 https://www.amazon.com/dp/B00BZ0O1O0/ref=cm_sw_r_awd_S-brwbM140M2R
This should do the trick. I had the same issues as you and this fixed it.
'06 GMC Duramax 4X4
'12 Fuzion 302
Winter 3 sleds
Summer 3 quads