[DeTomaso] ZF problems - need advice please

[Matt Enzed]

Ever since I got my Pantera, the ZF has always been problematic shifting into reverse, especially once
 it has warmed up.  Since I don't use reverse much, I didn't get overly concerned with it, and the only thing I did for the ZF was to change the

 gear oil once (last summer).  I used Valvoline 80W gear oil, not the 
recommended RedLine product, and did not see an improvement.  If 
anything, things got slightly worse.
Recently I started to have problems getting into the forward gears as well (once the ZF is hot). 
 When the transmission is cold, there are no problems. Also, when the car is 
warm, I have to turn off the motor to get it into reverse without 
grinding the gears.  Do you guys have any advice, short of pulling the 
ZF and sending it off for a complete rebuild, which I understand is very
 expensive? Should I change the fluid again, and use redline this time?