2004 GMC 2500HD LLY Duramax w/ Allison 254,000 miles. Slams into gear when going from park into neutral or drive. Also slams into every gear when it shifts. Upshifts and downshifts fine otherwise. No codes in transmission. Reset adaption..nothing changed. Oil is good and inside of pan is clean. Truck was just acquired so there is no history. Checked wiring from trans to TCM and even tried another valve body. Any suggestions?