gearvendor overdrive unit problems

[Portschy]

has anyone had or heard of problems with a gearvendor unit slipping (like an automatic trans slipping or a clutch slipping) i have a customers 96 3500 4X4 263,000 miles, in the shop & it was acting like a slipping clutch so we replaced it with the same results, found that the slip occurred after you let off the throttle & then back on, while it is slipping i can shift the unit into high range & in about 1 second it will grab & take off, i can get it to slip in high range but it slips about 3 times worse in low range, any information would be greatly appreciated, thanks mike e mail me at mportschy@hotmail.com