I know that the 5th gear unit is separate from 1-4, and have read in a few places that it is a weaker gear. Best thing I could tell you is to not lug it in 5th. Keep your RPM's over 1500 in 5th, and downshift it need be
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94 12V Reg Cab 2WD - NV4500
4" FBD straight, #100, 3GSK, Autometer Pyro/Boost