F150 Reverse Issue

I have a 2005 Yamaha F150 with about 200hrs on it and last week when I put it in reverse it started to grind. I was out all day and the boat shifted fine in both fwd and reverse. Then it would not engage in reverse. I looked at the linkage and it seems to be shifting and engaging properly when the motor is off. When the motor is running it will shift and operate fine in fwd but will just grind in reverse. I had the lower unit oil, water pump, and other normal maint done a month ago with Yamaha parts. The motor is still under warranty, but last time I had a warranty repair for the cylinder head it took 3.5 months to get it finished.

1) What easy items could I check?
2) Where could I get a used lower unit in case I have to get it repaired and it takes a long time?
3) Is this a common problem?

Thanks,

[jb123]

unhook gear cable at motor and push linkage in and out of gears via hand while running on hose to see if it is a simple cable adjustment.
.

Thanks for your help.

I unhooked the cable, fired up the motor on the hose, and shifted it in and out of gear. All worked fine so I adjusted the cable and took it for a sea trial. When the boat is in the water it will just grind and not go in reverse. I pulled it out and it would go in gear. Then back in the water, took the cable off and shifted and it would only grind. Pulled it back out and there are some metal shavings in the gear oil.

1) How hard is it to rebuild one of these?
2) Can I buy a rebuilt one?
3) Maybe I will just go without my boat while it is rebuilt under warranty even though it is prime season. The shop here has a 5 week wait just to get it in.