4L80E overdrive problems-Advise please!

[Thor]

I have a 94 Chev K2500 with the 4L80E transmission. It has 300,000 on the original transmission. It has never shifted properly into overdrive in 10 years, but always shifted. I have changed tranny filter and fluid avery 40,000 since I've owned it. It does not want to shift OD under load, had shop scan it, no codes, had a 068(tansmission slipping code in the history. Want to sell the truck, but want it to be working properly. Reverse, 1st,2nd.3rd shift fine. Is there anything I can check or replace externally or by dropping the pan? Thanks

[zijmer]

code 68 means to computor did'nt see a rpm change when shifting into 4th. problem could be a damaged over run sprag in the coast drum.