Chances are you will need to replace the unit. BTW I got one for my 87 745T transmission conversion for $250 off another Volvo owner. I ran your request through www.car-part.com and got prices from $200 to $850.
I cooked two transmissions while under warranty with the same symptoms as you are having. I learned under Volvo's Buck with my 86 740 not to put the clutch in and shift into a lower gear while coasting. It sounds like the syncros are shot in your first and second gears. In my case I also cooked the bearings.
Both times Volvo opened up the transmission, the gears were perfect there just was tiny pieces of brass everywhere inside the case. They never figured out what was happening but I did.