Jeff,

Does your 3604 truck have a totally enclosed driveline? My first thought would not be the tranny but rather a u-joint that was near failure. For the transmission to act this way I think you would have evidence of a terribly sloppy shifter and some type of gear clanging or bearing whine.

I wouldn't rule out a ring and pinion problem either. If the transmission has a band style parking brake, give that a inspection too. I think the tranny guts would be my last choice though.

Stuart