700r problems with reverse

[Ehodg66]

I have an 82 C10 w/ a 350 crate and 700r trans. The PO did a frame off build roughly 6 years ago. The odometer currently reads 55k (I've put roughly 10k on it) but I have no idea if the odometer was set to zero at the rebuild. Also, I do not know if the tranny was rebuilt or how many miles were on it before being installed in this truck.
Started my truck this morning (daily driver) to let it warm up, as I do every morning. I put it in reverse and nothing. It works in drive and the lower gears. I tried reverse again, went 6" and stopped. Restarted the car, tried reverse again, it started to go back but the tranny acted as if the engine was stalling (if that makes sense). I put it back in drive, and it did the same thing, not smoothly moving forward.
Prior to this I've had no problem with shifting gears or the tranny making odd noises or acting strange at all. It was cool here in south Florida this morning but was colder a few weeks ago and did not have this issue.
I know I will be taking this to a shop but just looking for some insight of what I MAY be looking at ahead.

Thank you in advance