4 spd on the floor in a 66 C10 popping out of gear

[McLovin]

2nd gear in my 4 spd transmission is popping out of gear. It happens during acceleration from a stop or when I am just cruising at low speed at light throttle. This is a 4 spd with granny low. Numbering the granny low gear as 1st gear it is 2nd gear that is causing me problems. I can hold it in gear if I hold down on the shifter but that doesn't always work. Local trans shop told me he thought it was the bearings and quoted me 650 bucks to fix if I pulled the trans out and took it to him.

Like most people with this trans I usually start from a stop in second gear, not really using granny gear unless I need to go really really slow...

Anyone have any ideas as to what is wrong? Is it the bearings? Anyone have a spare 4 spd in Oregon they are willing to part with?

Any help would be appreciated... Thanks!